Maybe the IED was part of plan B, or Plan C, in case he got made before he got out of the van. I'd have to think you'd expect the distinct possibility that you'd be found out before you even got parked. Even if the kid had made one or more recon trips to the site, days, or weeks before, in the middle of the night, scouting the area, he'd have no real way of knowing how difficult it might be to execute any plan on the day of the event. I wonder if he was genuinely surprised that he actually made it to his spot with time to shoot.Found an IED in the vehicle. This is a bit more conspiratorial than usual, but was the plan to get off the roof and go back to the vehicle? (tin foil hat on) Would the IED have detonated if he returned to the vehicle? Bad guy gets smoked by his own IED, materials match what is found in the home. Case closed.
If the would-be assassin was successful, but got blown up by his own IED. Lone wolf who was bullied in school, received a STEM award upon graduating HS (mentioned a number of times) so making explosives is not a reach, neither is getting himself blown up. Only 20 year old without a huge electronic footprint.
Seems like a bit of a stretch, but so does just showing up with the right size ladder to get on a roof 130 yards away.