For a last ditch effort, you could try to look up the laws in the early 1800s (1818?) or whenever the book is actually set, to see about negligence laws. Most likely, it was lax or nonexistent, so your mock court proceeding shouldn't be taking place at all. And if they say that they are having a modern court proceeding, bring up statute of limitations since the actions occurred.

It has been ~205 years since my client allegedly committed this crime, ~185 years past the statute of limitations or whatever if he is guilty of anything. Also a funny ending could be saying that your client has been dead for hundreds of years, and thus cannot be charged.

These probably don't really help you that much.

You could argue that instead of acting in disregard for obvious risks to human life and safety, he does the exact opposite. He protects humanity by refusing to create a second supernatural being from coming to life that could threaten thousands of lives.

Plus, right when the being is created, there's no way of knowing if it's actually living. Living means the seven criteria in biology that we specify, so you could maybe refute that Frankenstein is indeed a living being since it cannot reproduce (maybe??).

Along the same vein, right when it awakens, there's no way to know its intelligence, capacity to understand human language, speak, or possible danger to humanity. It could've stood up, walked over to him and then fell over dead again, how was he to know when the galvanization would stop working? We find out later that the monster can jump great distances, run at a speed much faster than humans, has superhuman strength, and withstand frigid temperatures well. Maybe it is even a different species at this point, and that could nullify the negligence part for Victor as creator/parent/assumed guardian.