A suicide mission against an armed guard or an armed self ended in suicide.<quoted text>
I don't have to guess that he chose an unguarded school and attacked the most defenseless crowd he could find. Then, instead of waiting to fight it out with the armed people that would inevitably blow him into the next dimension, he kills himself.
Case closed. His target was the most defenseless victims he could find, and the prospect of him actually having to fight it out with people equally well armed caused him to kill himself before the fight started. So, any body armor he may have worn was just part of his costume. He had no intention of putting it to its intended use.
Just the presence of an armed guard at that school would have prevented this.
According to reports, he had been in a conflict with 4 teachers at the school the previous day, and that was far more likely to play the major part in his choices than just selecting a nice venue for a day's outing of recreational mass murder.