re: Why is Alec Baldwin responsible for murder?

There’s criminal intent, and then there’s criminal negligence. The first would be specifically intending the consequence of death by shooting that gun. Criminal negligence would be acting in such a dangerously careless manner that one eithef knew or should have known that a death could by way of his actions. Intent carries the stiffest of penalties, while negligence has a pretty wide ranging penalty but on the lower end.

I could see negligent homicide being the case, but definitely not intentional murder, unless some wild and unlikely new evidence crops up.