This is funny:

Ran into a legenary protectron in Kellog's house that would cause me to blow up and die everytime I killed him. I was going crazy the first few times it happened.

* First couple of times it happened I thought I stepped on a mine and blew up - which was weird because I have the Sneak perk for it. However, I combed the area and saw nothing.

* Next few times, I figured, well he's legendary so maybe he blows up and I'm just a bit too close. I tried running away and still blew up.

* At this point, I assume that he kills whoever kills him no matter what, and that seemed to be the case. I died every single time I killed him, so I reloaded and ran passed him.

I did not try having my companion kill him, nor did I try an indirect approach like a mine. My suspicion is that he can only be safely killed by shutting him down via the Robotics Expert. Lesson learned: don't @#$% with legendary protectrons.