His first mistake (besides pissing off the person who ratted him out) was to answer his door holding a gun. It sounds to me like he was anticipating cops showing up to grab his guns. Reminds me of suicide by cop. Now if the cops had body cameras on the footage from those would be instructive. Personally, I think this law is bullshit and I live in Maryland. If I had any guns, no one other than one or two trusted relatives would know and any government certainly would not. But when you are outgunned and pinned in your own home, the sane person would acquiesce.
I do not think it is abnormal to answer your door with gun nearby at odd hours of the day.