A police sniper would very likely not be authorized to kill a barricaded shooter who wasn't actively in the process of threatening a civilian or officer. "Imminent threat" is an important standard.

If they had time to rig up an IED to the robot, and drive the sucker in there, use the camera to make sure he didn't have any hostages (they did that right?), and THEN detonate the bomb, it doesn't sound like the threat was all that imminent.