Premeditated because he had a loaded gun and a .357 isn't used to shoot flies. He was expecting visitors and it was clear he was ready to do whatever to fend them off.
He didn't know they were cops? They usually barge in yelling POLICE like crazy! This is part of the intimidation tactics they use. Also, if the cop was hit in the head, when you compare the size of a head with the rest of the body, the guy was definetly aiming to give no chances...
The kid might have dialed 911 but, if he knew nothing of his dad's business, it's expected. Doesn't prove anything at all.
If the gun was registered, what in hell the guy doing with a .357? How was he able to register it? It was loaded so, chances are very high the guy carried it with him, registered or not so, catching the guy would not have changed anything, beside putting neighborhood in danger.
Somebody with a clear conscience doesn't keep a .357 loaded in a house where there's kids. Only reinforce he had it on him and knew he might get visitors.
These parents should loose their kids forever for having put them in a position like this. You want to deal drugs? Don't live with kids and even worst, with a loaded gun. The mom isn't better than the dad, no matter if she was invlolved or not, she let the gun in and tolerated the guy's business without leaving.