fifty pounds? That’s a lot of tannerite. Perhaps to bomb the first responders while he made an escape? Attacking the first responders is a Al Quaeda/ISIS tactic.
CC
If the tannerite was already mixed and he could see his car from his room, maybe he was going to shoot into the car and create a giant explosion as a distraction, or to kill more people. If that’s what happened, they are lucky he didn’t get around to it.