He wouldn't need body armor for a theater full of unarmed people, he would need it for a shootout with police to get into the theater.
I don't see that forethought as the thinking of one person.
Some deranged killers might get off on the planning part. The Zodiac killer planned his deeds in a way that he never was caught. As for the body armour and planned shootout with police, when given the opportunity, the maniac threw his hands in the air and gave up. I would guess the costume was more for effect than to be used.