You can kill Spot. This happened to me in my current game. We are playing 2 player and the other guy is a dark urge. He was having fun with the dialogue when using Speak with Animals with Spot and turned him hostile to our party. My Paladin attacked first and killed Spot with one hit, losing her oath in the process (my Paladin is Shadowheart). It actually seems fitting for her as a wan-a-be Dark Justiciar.

"He that can smile at death, as we know him. Who can flourish in the midst of diseases that kill off whole peoples. Oh! If such a one was to come from God, and not the Devil, what a force for good might he not be in this old world of ours."
-Bram Stoker, Dracula