He wanted to kill a lot of people and get away. The tools, change of clothes, and black powder all support that hypothesis. What's conspicuously missing from the list are the flex-cuffs or wire ties that we know he had.
At one point it was stated that they found a backpack belonging to him, and cleared out the school. I suppose this is what was left inside the backpack, in addition to the weapons. I don't know however why they would leave out the ties and flex-cuffs.