Frankenstein: Justine Moritz's trial

I'm reading Frankenstein, the 1818 text. What can I say now that hasn't been said about Mary Shelley's novel? Perhaps nothing. 
Anyway... 
Here are the 1st details that tie Justine Moritz to the murder of William: 

"... the morning on which the murder of poor William had been discovered, Justine had been taken ill, and confined to her bed; and after several days, 1 of the servants, happening to examine the apparel she had worn on the night of the murder, had discovered in her pocket the picture of my mother, which had been judged to be the temptation of the murderer. The servant instantly shewed to 1 of the others, who, without saying a word to any of the family, went to a magistrate; and, upon their disposition, Justine was apprehended. On being charged with the fact, the poor girl confirmed the suspicion in a great measure by her extreme confusion of manner..." 

At court: 

"Several strange facts combined against her, which might have staggered anyone who had not such proof of her innocence as I had. She had been out the whole of the night on which the murder had been committed, and towards morning had been perceived by a market-woman not far from the spot where the body of the murdered child had been afterwards found. The woman asked her what she did there; but she looked very strangely, and only returned a confused and unintelligible answer. She returned to the house about 8 o'clock; and, when one inquired where she had passed the night, she replied that she had been looking for the child, and demanded earnestly if anything had been heard concerning him. When shown the body, she fell into violent hysterics, and kept her bed for several days. The picture was then produced, which the servant had found in her pocket; and when Elizabeth, in a faltering voice, proved that it was the same which, an hour before the child had been missed, she had placed round his neck, a murmur of horror and indignation filled the court." 

Here is her own defence: 

"... She then related that, by the permission of Elizabeth, she had passed the evening of the night on which the murder had been committed at the house of an aunt at Chene, a village situated at about a league from Geneva. On her return, at about 9 o'clock, she met a man, who asked her if she had seen anything of the child who was lost. She was alarmed by this account, and passed several hours in looking for him, when the gates of Geneva were shut, and she was forced to remain several hours of the night in a barn belonging to a cottage, being unwilling to call up the inhabitants, to whom she was well known. Most of the night she spent here watching; towards morning she believed that she slept for a few minutes; some steps disturbed her, and she awoke. It was dawn, and she quitted her asylum, that she might again endeavour to find my brother. If she had gone near the spot where his body lay, it was without her knowledge. That she had been bewildered when questioned by the market-woman was not surprising, since she had passed a sleepless night, and the fate of poor William was yet uncertain. Concerning the picture she could give no account..." 

Have I said that I love courtroom films? I do. This case, I initially thought, is mostly based on circumstantial evidence, except for the picture. It makes me think of The Moonstone, in which Sergeant Cuff's conclusion about Rosanna's guilt, based on facts, observations and logical reasoning, makes lots of sense, though it's wrong. Then it's hard to say, the verdict is understandable. The case seems convincing enough, especially because Justine later confesses. Why does she, if she's innocent? The people who believe her to the end do so mostly because of their confidence in her character, in her goodness, and who knows, they may be wrong. I don't know anything- of course Frankenstein's 1 of those famous novels of which (almost) everyone knows something (like Dracula, Strange Case of Dr Jekyll and Mr Hyde, Pride and Prejudice, Anna Karenina...), but I know the general plot, not the details, and don't remember having watched any film adaptation (if I have, it must have been a long time ago). Anyhow, that's beside the point. What I mean to say is that, I'm not making any guesses at this state about Justine. The question is: why is Victor Frankenstein absolutely convinced that his monster's the true murderer? What does he have?

"What did he there? Could he be (I shuddered at the conception) the murderer of my brother? No sooner did that idea cross my imagination, than I became convinced of its truth; my teeth chartered, and I was forced to lean against a tree for support. [...] Nothing in human shape could have destroyed that fair child. He was the murderer! I could not doubt it. The mere presence of the idea was an irresistible proof of the fact..." 

He doesn't even have circumstantial evidence! Where does that conviction come from? The monster's created on page 39 in my book. In shock Frankenstein goes away, and upon coming back, finds it gone. The monster appears again on page 56, and disappears. Right now I'm on page 69, there has been nothing new between the creator and the creation. Why is he so sure?