If you look at the time line, it seems to me that he was planning on having at most some one with a CCW and pistol rather than the police with more fire power. The music in his apartment went on around midnight. The downstairs neighbor went to investigate and complain but didn't open the door. If she had set off the bomb or the police investigating the noise complaint, every first responder in town would have been at or on the way to the scene of the bomb, leaving the theater even less protected.
Some times you are left with only bad and worse choices. But even in body armor he would have been vulnerable to a head shot or at least being distracted long enough for people to physically restrain him. As you say "Practice, Practice!