Because you're a threat. Whatever last minute save your own skin decision you made once it became obvious you were going to lose means nothing. You still nearly killed our men, attacked them unprovoked, and stood by idly as your psycho she-witch master murdered some of our best and plotted to kill every human on the planet.
Tell yourself what you want, but no one is stupid enough to "think it needed to be done" without having that one dark spark of genocide in them.
But what do I know? Go ahead and listen to Bonnie and Clyde there. You'll make a wonderful threesome.