"[H]e must die the death and be murdered, in order to light the gay bridals and other merry-makings of men, and also to illuminate the solemn churches that preach unconditional inoffensiveness by all to all"- The whale-killing scenes in Moby Dick

When we finish chapter 81 "The Pequod Meets the Virgin" of Moby Dick, the Pequod has killed 3 whales. However, this chapter is different. 
The 1st whale is killed in chapter 61 "Stubb Kills a Whale". Ishmael's standing at the foremast-head, watching; the whale suddenly appears. 

"And lo! close under our lee, not forty fathoms off, a gigantic Sperm Whale lay rolling in the water like the capsized hull of a frigate, his broad, glossy back, of an Ethiopian hue, glistening in the sun's rays like a mirror. But lazily undulating in the trough of the sea, and ever and anon tranquilly spouting his vapoury jet, the whale looked like a portly burgher smoking his pipe of a warm afternoon. But that pipe, poor whale, was thy last." 

That last sentence is a sad one, because the pipe seems to represent, or can be understood as, an enjoyment of life (the happy-go-lucky Stubb always has a pipe in his mouth; Ishmael and Queequeg reach a kind of closeness sharing a blanket and a smoke; Ahab throws away his pipe because he no longer has the serenity to enjoy it...), and the idea is made even stronger because it's not a real pipe, but a "pipe of a warm afternoon". 

"After the full interval of his sounding had elapsed, the whale rose again, and being now in advance of the smoker's boat, and much nearer to it than to any of the others, Stubb counted upon the honour of the capture. It was obvious, now, that the whale had at length become aware of his pursuers. All silence of cautiousness was therefore no longer of use. Paddles were dropped, and oars came loudly into play. And still puffing at his pipe, Stubb cheered on his crew to the assault.
Yes, a mighty change had come over the fish. All alive to his jeopardy, he was going "head out"; that part obliquely projecting from the mad yeast which he brewed." 

Here Ishmael focuses on the crew members (Stubb, Queequeg, Tashtego), on the process of capturing the whale (method, actions, tools). Being attacked, the whale starts bleeding. 

"The red tide now poured from all sides of the monster like brooks down a hill. His tormented body rolled not in brine but in blood, which bubbled and seethed for furlongs behind in their wake." 

Stubb churns his lance into the whale. 

"... When reaching far over the bow, Stubb slowly churned his long sharp lance into the fish, and kept it there, carefully churning and churning, as if cautiously seeking to feel after some gold watch that the whale might have swallowed, and which he was fearful of breaking ere he could hook it out. But that gold watch he sought was the innermost life of the fish. And now it is struck; for, starting from his trance into that unspeakable thing called his "flurry," the monster horribly wallowed in his blood, overwrapped himself in impenetrable, mad, boiling spray, so that the imperilled craft, instantly dropping astern, had much ado blindly to struggle out from that phrensied twilight into the clear air of the day.
And now abating in his flurry, the whale once more rolled out into view; surging from side to side; spasmodically dilating and contracting his spout-hole, with sharp, cracking, agonized respirations. At last, gush after gush of clotted red gore, as if it had been the purple lees of red wine, shot into the frighted air; and falling back again, ran dripping down his motionless flanks into the sea. His heart had burst!" 

It's a striking image ("red tide", "gush after gush of clotted red gore", etc.), and Ishmael does write "tormented body" and "agonized respirations", but in this chapter he's rather neutral and objective- he's just describing everything he sees and helping us visualise how a whale is captured. The focus of the chapter is not on the whale, but on the whalemen. Ishmael goes on to devote the 2 next chapters to the tools used in the capture. 
The 2nd whale-killing scene is found in chapter 73 "Stubb and Flask Kill a Right Whale; and Then Have a Talk". It is brief. 

"An interval passed and the boats were in plain sight, in the act of being dragged right towards the ship by the towing whale. So close did the monster come to the hull, that at first it seemed as if he meant it malice; but suddenly going down in a maelstrom, within three rods of the planks, he wholly disappeared from view, as if diving under the keel. "Cut, cut!" was the cry from the ship to the boats, which, for one instant, seemed on the point of being brought with a deadly dash against the vessel's side. But having plenty of line yet in the tubs, and the whale not sounding very rapidly, they paid out abundance of rope, and at the same time pulled with all their might so as to get ahead of the ship. For a few minutes the struggle was intensely critical; for while they still slacked out the tightened line in one direction, and still plied their oars in another, the contending strain threatened to take them under. But it was only a few feet advance they sought to gain. And they stuck to it till they did gain it; when instantly, a swift tremor was felt running like lightning along the keel, as the strained line, scraping beneath the ship, suddenly rose to view under her bows, snapping and quivering; and so flinging off its drippings, that the drops fell like bits of broken glass on the water, while the whale beyond also rose to sight, and once more the boats were free to fly. But the fagged whale abated his speed, and blindly altering his course, went round the stern of the ship towing the two boats after him, so that they performed a complete circuit.
Meantime, they hauled more and more upon their lines, till close flanking him on both sides, Stubb answered Flask with lance for lance; and thus round and round the Pequod the battle went, while the multitudes of sharks that had before swum round the Sperm Whale's body, rushed to the fresh blood that was spilled, thirstily drinking at every new gash, as the eager Israelites did at the new bursting fountains that poured from the smitten rock.
At last his spout grew thick, and with a frightful roll and vomit, he turned upon his back a corpse." 

Again, Ishmael's just describing. The death of the right whale, in itself, isn't important- it's what it means that is. The chapter's to show Ahab's madness and Fedallah's mysteriousness, at least from the point of view of Stubb and Flask. 
It's different when we get to chapter 81. Here, what we have is "a huge, humped old bull", whose "spout was short, slow, and laborious; coming forth with a choking sort of gush, and spending itself in torn shreds". The whale has 1 fin. 
Ishmael describes the competition between the 2 ships, and then: 

"It was a terrific, most pitiable, and maddening sight. The whale was now going head out, and sending his spout before him in a continual tormented jet; while his one poor fin beat his side in an agony of fright. Now to this hand, now to that, he yawed in his faltering flight, and still at every billow that he broke, he spasmodically sank in the sea, or sideways rolled towards the sky his one beating fin. So have I seen a bird with clipped wing making affrighted broken circles in the air, vainly striving to escape the piratical hawks. But the bird has a voice, and with plaintive cries will make known her fear; but the fear of this vast dumb brute of the sea, was chained up and enchanted in him; he had no voice, save that choking respiration through his spiracle, and this made the sight of him unspeakably pitiable; while still, in his amazing bulk, portcullis jaw, and omnipotent tail, there was enough to appal the stoutest man who so pitied." 

If the whale-killing scene is just a job, a process in chapter 61, and a meaningful, symbolic act in chapter 73, the one in chapter 81 is more. The narrator now concentrates on the whale, on the pain of the whale. 

"As the three boats lay there on that gently rolling sea, gazing down into its eternal blue noon; and as not a single groan or cry of any sort, nay, not so much as a ripple or a bubble came up from its depths; what landsman would have thought, that beneath all that silence and placidity, the utmost monster of the seas was writhing and wrenching in agony!" 

Those passages are sadder than anything about whaling that we've seen in Moby Dick. The narrator now no longer sounds like a whaleman or a member of a whaling ship, he sounds like a spectator shocked by the awful scene he witnesses. Does Ishmael now doubt the job he has chosen, and start to question the point of killing whales? 
He doesn't stop there. He goes further. 

"As the boats now more closely surrounded him, the whole upper part of his form, with much of it that is ordinarily submerged, was plainly revealed. His eyes, or rather the places where his eyes had been, were beheld. As strange misgrown masses gather in the knot-holes of the noblest oaks when prostrate, so from the points which the whale's eyes had once occupied, now protruded blind bulbs, horribly pitiable to see. But pity there was none. For all his old age, and his one arm, and his blind eyes, he must die the death and be murdered, in order to light the gay bridals and other merry-makings of men, and also to illuminate the solemn churches that preach unconditional inoffensiveness by all to all. Still rolling in his blood, at last he partially disclosed a strangely discoloured bunch or protuberance, the size of a bushel, low down on the flank." 

Aren't human beings selfish? As though that isn't enough, Ishmael describes Flask pricking the whale again, for the mere fun of it. 

"At the instant of the dart an ulcerous jet shot from this cruel wound, and goaded by it into more than sufferable anguish, the whale now spouting thick blood, with swift fury blindly darted at the craft, bespattering them and their glorying crews all over with showers of gore, capsizing Flask's boat and marring the bows. It was his death stroke. For, by this time, so spent was he by loss of blood, that he helplessly rolled away from the wreck he had made; lay panting on his side, impotently flapped with his stumped fin, then over and over slowly revolved like a waning world; turned up the white secrets of his belly; lay like a log, and died. It was most piteous, that last expiring spout. As when by unseen hands the water is gradually drawn off from some mighty fountain, and with half-stifled melancholy gurglings the spray-column lowers and lowers to the ground—so the last long dying spout of the whale." 

Chapter 81 is the saddest chapter in the book so far. If before Ishmael watches from afar, now he comes close to the whale, and seems to feel what the whale feels. He anthropomorphises the whales. 

"Usually the dead Sperm Whale floats with great buoyancy, with its side or belly considerably elevated above the surface. If the only whales that thus sank were old, meagre, and broken-hearted creatures, their pads of lard diminished and all their bones heavy and rheumatic; then you might with some reason assert that this sinking is caused by an uncommon specific gravity in the fish so sinking, consequent upon this absence of buoyant matter in him. But it is not so. For young whales, in the highest health, and swelling with noble aspirations, prematurely cut off in the warm flush and May of life, with all their panting lard about them; even these brawny, buoyant heroes do sometimes sink."

The chapter is so sombre, and Ishmael appears so critical of whaling, that right after this chapter comes "The Honour and Glory of Whaling". But does whaling sound honourable and glorious any more, after such a scene?