I would say that in a case like that we would still be well within the bounds of the existing legal frameworks to assign blame according to contributing factors

I would say that in a case like that we would still be well within the bounds of the existing legal frameworks to assign blame according to contributing factors, strength of causality, etc. There's no legal principle that only one person can be criminally liable for murder. Therefore, if the AI is a legal person, it could be criminally liable. If its designers designed it with murderous intention, or reckless disregard for life, so could they. It would just depend on the circumstances, as would the punishment. I'm sure the law would need to be updated to take into accounts the rights of created persons, if we ever get to that point, but it doesn't seem remotely insurmountable.