I think it's a mistake to come from the standpoint that "the charge mistakenly likens the Monster to a human child". I think the charge doing that is in your favor. If you try to refute that idea it'll get turned around on you - so make "The charge likens the Monster to a human child" the argument that is in your favor. Start here -

If the charge does assume that the Monster is like Frankenstein's child, then we must also ask ourselves a question about fathers and parents - when is it that a parent is no longer culpable for the actions of the child? It could be argued generally that this happens whenever the child reaches the "age of reason", whatever that might be for the culture in question. In our culture, that equates to the teen years sometime, but the actual age is not important - it's the autonomy that is gained. When one becomes an adult and stops being a child/minor/whatever, they gain personal autonomy.

Was the Monster acting with autonomy? We have to conclude that he was. He made the conscious decision to do the things that he did with direct influence or instruction from any outside party. Not only that, but he did those things while sustaining his own existence. Did Frankenstein abandon him, like so many fathers abandon children? Perhaps, and perhaps what he did was even worse than that (because Frankenstein is definitely morally reprehensible - you can't avoid that and should avoid trying to make him the "good guy" at any point). But if the Monster was a helpless dependent, then he shouldn't have even survived without Frankenstein. He not only survived, but he learned to speak and read while in hiding, from watching people. He was isolated, but he thrived and grew - all on his own. Did the father influence the child? Even with nothing more than absence this is inevitable - parents always influence their children. But did the father instruct or force the child - the Monster - to do what he did? No - he did that on his own completely.

Further, in his own narrative the Monster says that he initially wanted to connect with humans. It was only after mistreatment after mistreatment that he decided that he was going to start killing humans. If Victor was responsible, why didn't the Monster immediately begin killing after Victor abandoned him? The answer is because the Monster chose to do those things, and Victory Frankenstein was not involved in that decision. He created the person that made the decision, but he did not motivate it.

Finally: Did Victor Frankenstein intend to create a creature which would cause death? Absolutely not. His intentions were... sort of good. His real intention was to self aggrandize and be a little god. Still, at least he wanted to do so by creating, by bringing forth life from death. It was the Monster, while isolated completely from it's "parent", who decided that humans did not deserve to live. What did Victor Frankenstein ever do that would motivate the Monster to kill? Well... I guess the only thing would be that Victor did abandon him. But every single person the Monster meets rejects him. If Victor Frankenstein is responsible for the Monster's actions, so are we all, every single one of us who would run in fear at the sight of him. And I know I'm not responsible, and I don't think you are either - so that means Victor Frankenstein isn't, just the same.

Anyway, I'm rambling, but I hope this is cogent and maybe even helps. smoke bomb