Was using the Dallas robot bomb legal?

A T-800 Terminator in a scene from "Terminator Salvation," a Warner Bros. Pictures release. I hope the Dallas robot bomb isn't quite that….

Gizmodo (Michael Nunez) has some reactions to the news that the Dallas shooter was killed by a remote-controlled bomb-carrying robot (apparently a first in American policing). My view on the legal question, which the story quotes: If the police reasonably believe that someone poses an imminent danger of death to others, and that killing him is necessary to prevent that danger, they can try to kill him, whether with a rifle or a bomb-carrying robot.

Jay Stanley of the American Civil Liberties Union, quoted by Gizmodo, agrees, though he adds that

the easy and relatively safe use of ground robots that can deploy deadly force could mean they could be overused: "Remote uses of force raise policy issues that should be carefully considered … and should remain confined to extraordinary situations," he said.

I in turn agree with that - reason for caution, but not, I think, reason for avoiding this technology altogether.