Proponents of the death penalty would disagree with you.
Nice try, but different situation. Presumably an objective and fair trial and sentencing took place. And we've already discussed the situation where the party knows the law is corrupt and they won't receive a fair trial, and they turn them over to them anyway.
But, in the process of responding, it did highlight a problem with the presumed definition of evil (which is related to the L5R tangent).
Here's one example - Killing an intelligent, non-evil opponent is not evil when done:
In self-defence
To protect another
In times of war, against enemy combatants (unless capture is a viable option).
I would hope most of us would agree with this, although the third point leaves a lot of room for interpretation.
It's clear that most of us agree that the OP where the character killed a man who was not an enemy combatant (as in the enemy in a war), who was unarmed and surrendering, is an evil act.
The fact that some question that or attempt to justify first really surprised me. When I read the post, I honestly found it surprising that it was even asked, but didn't think my "me too" response that it is obviously an evil act would be needed. It was when people questioned it was when I felt I needed to reply.
The reality is, any definition other than, "killing another intelligent creature is evil" has justifications for the times that it's not evil to kill the same being. Which isn't really a problem, since that's the situation in our own world. There are people who agree with the death penalty, and those that don't. There are pacifists who don't believe war, or even self defense is sufficient. There are vegans that don't believe killing any living creature is acceptable (although ironically some of them agree with the death penalty or going to war).
And this could actually be an interesting thing between characters.