Good question. I don't know, but I think your speculation might be correct. He probably wasn't real bright to begin with, and mean as a snake on top of that. I think most likely he started the fire to cover up the murder, then when the firefighters showed up, it escalated from there. Might not have originally intended to commit suicide. Who knows.
I know, except it “says” the fire was meant to lure firefighters...although how can they know that?
The impression they wanted to leave was that he planned to shoot them after luring them there with the fire.
Your scenario has a gradually unfolding event with a not-too-bright, and mean, murderer (having killed his own grandmother) sort of winging it after starting out killing his sister.
The pattern of murdering family members seems clear. What isn’t yet clear is if he meant to commit suicide after he murdered his sister, meant to shoot the firemen etc or was this something that escalated.
The murderer had an arsenal of weapons ready for the arrival of any first responders, apparently, so if only from that, I’m leaning that he meant to do the entire thing from the beginning.
If true, then I don’t think the fire was to cover up a crime, since he was planning to kill himself at the end.
Seems like he was a destroyer...destroyed his grandmother, his sister, the house and the first responders he shot. And everyone connected to them...
Got his name up in lights, too, which was probably something he desired since he was a total loser otherwise...
Never try to get inside a mad man’s head and attempt to understand what you find there, for if you do understand what you find there, you’re just as crazy as he is.