The suspect has not totally disregarded his fingerprints. The way he planned everything, I just don't see it. Obviously he wanted that backpack to be found, so he isn't going to leave prints on it or the energy bar that is in the pocket. He covers them with a clear silicone that dries so they don't show up.
Going to another level of murder mystery writing: he brings with him that morning an empty water bottle and the empty wrapper that somebody at Starbuck's had thrown away on a previous day. He grabbed them out of the trash. He specifically goes to Starbuck's to buy those things before the shooting and puts them in his pocket, but places the ones with somebody else's prints/dna on them near the scene.
LE finds those items after the shooting, all excited that the found gold. The results come back. The person who drank the water and ate the energy bar at Starbuck's name was...
Brian Thompson.