That's a far easier scenario than the ones involving recursively generated AIs. You are, and would likely be charged with Murder 2 in most US jurisdictions.

Wrong analogy. I set up a blank slate. Random people on the internet start to mould it into a gun that fires on people incrementally, each step fairly innocuous. Who is responsible?

"who is responsible?" is a philosophical question that probably has no objective answer.

"who would be held responsible?" is still pretty easy here. by default, if you own or release to the public a system that ends up hurting someone, you are liable. it doesn't matter what your intent was or whether there was any way of knowing that the bad outcome would happen. the only way you can get around this is with "safe harbor" exemptions. it's possible that your blank slate would qualify, although you would probably have to lobby for this before the fact. in that case, no one would be held responsible. otherwise, you would be paying.

The community has a garbage tip. Due to an abundance of uraniaum, everyone starts discarding it. When the garbage tip reaches critical mass and kills everyone, who was to blame?