Yeah, it's negligent homicide at the very least. One is guilty of negligent homicide when one is doing something they are legally entitled to do but they do it negligently resulting in death. There is no way Baldwin didn't know this was a real gun, there is no way he didn't know that there is a possibility of harm or death from anything that even looks like a real gun. He has been through the whole gun safety thing hundreds of times before on other sets, he has been in dozens of films with scenes involving firearms. There is no excuse.
He's lazy, he's stupid and he hates guns so it was just too much trouble for him to check it himself.