What if they called it manslaughter instead? I mean... I kind of get where you're coming from, that the other guy didn't intend to kill Cooper, but they did intend to perform a crime, and that crime came with the potential immediate penalty of death (as was the case for Cooper) should the homeowner defend themselves.
I think it's only fair that the guy who lived suffer along with Cooper's death. I mean, he could just have easily gotten killed himself with no penalty to the defenders (as there rightfully shouldn't be if someone is invading your home and threatening your well being).
I think the law Reba mentioned makes sense. If you're going to commit a crime that causes things to spiral out of control, you are directly responsible for the result and should be held to it by the law.
Thinking about it more, manslaughter makes sense, but murder fits too... It's like if you're driving recklessly, you don't intend to kill anyone, but if you do, you can be up for manslaughter or murder (I think the level of negligence, maybe remorse, and probably other stuff gets weighed in to determine the charge).