It's not evil, it's just not especially good.
A good character might have wanted to give the guy a chance to mend his ways. He didn't actually kill anyone, he just tried to, and there was a chance to take him out of the fight without killing him, so death is excessive from a good PoV. Batman, for example, wouldn't kill a thug who tried to kill him. Mercy would be the rule. Good characters kill only when necessary.
Just because the CDG isn't something a good character would do does not make it evil, however, and besides, any DM who tells you that you didn't do something is not the kind of DM you want to game with.
Rules wise, isn't there a thing you can do to a helpless opponent that's very much like a coup de grace in execution but actually knocks the opponent out cold for several hours, like how an assassin can use death attack to paralyse instead of kill?
If not, I'd suggest the following as a house rule: the knockout strike. Resolve exactly as a coup de grace, but with the attacker dealing non-lethal damage, and a failed save resulting in unconsciousness rather than death.