I am thinking this wasn't particularly planned, though it may have been a fantasy for a long time. He liked guns and purchased them, perhaps not with a particular goal in mind but just because he liked them, had the money, and knew how to get them. And, then, the fantasy was something he could actually accomplish rather than just think about, so he did it.
I don't see a particular planner here - just someone with guns who decided to kill and since he had guns, he did it.
(I think the smoke alarm went off because of gunpowder, not because he pulled it. Unless someone knows that couldn't happen, that's the scenario that makes most sense to me.)
jmo