It was rage.

It was rage that had driven Bushel to kill Fire Blast. Pure, unfiltered rage for all the unjust and evil deeds he had performed,all the atrocities and unpunished crimes. The earth pony's sins radiated off of him like an aura of absolute evil, and Bushel knew that if he didn't act now the monster in a pony's skin would only do more damage. So the unicorn charged his horn and did what had to be done, consequences be damned.