Hang on, you couped an evil human who tried to kill you, in the middle of a raging fight, who could very easily have woken back up to threaten you, and your DM says it's evil?
Evil would be cutting off his arms, legs, and tongue and letting him bleed to death in the wilderness. You can justify this in a number of ways:
1 - You defeated him in a fair fight (though if he ambushed you, it actually wasn't fair), and you made sure he couldn't come back to hurt you or your party, or for that matter, any innocents on the road that he would kill if you let him live.
2 - He tried to kill you. He failed. He shouldn't have tried to kill you.
3 - He's evil, and killing evil creatures is good, therefore killing him was a good act.
4 - He's in violation of the law, and he's attempted murder (and he's probably murdered in the past anyways), so you're simply enforcing justice in the simplest manner possible.
5 - He was a threat to the group's survival that needed to be put down to ensure your survival. Defending yourself isn't evil.