Why. Isn't that the question. If the user had the processing power to do anything else, this entire mess could have been avoided.

Or more likely, delayed. Rinzler's mask turns just a little to the side, but the stall doesn't last. He doesn't want to loop over the inevitabilities, and if he still can't parse why Chara cares about the motives of a corpse, he doubts they'll try to use the data against him.

User interfering with personal possessions (weaponry).
Confronted it.
User talked. Attempted distraction. Attacked.

There's a slightly irritated shrug, damage brightening on his left side with the motion. It was stupid. It was a user, and Rinzler shouldn't have killed it. But he's not sorry it's dead.