Perhaps you've missed my point. The notion that an action is immoral but right is verging on contradictory. (There are arguments to the contrary: say, dirty hands arguments; or the sorts of arguments that Raymond Geuss runs in Outside Ethics; but you are not running such arguments.)
What do you mean by "good reason"?
And for that matter, what do you mean by "evil act"?
Evil Act - Any act which, in isolation, would be considered immoral, ex. killing someone in cold-blood, intentionally inflicting excessive harm on another person.
Good Reason - Any rationale that the good being achieved justifies committing an Evil Act.
This is part of what I'm talking about when I say an Evil action can be right, the action being taken is evil but it's for good reasons. The other part is where the action may not be morally justified but is none-the-less correct. For example if a group of characters has taken an enemy prisoner who's bothered them for some time and decides that if they take him in he'll just escape and come back to pester them again, so they just kill him.
Of course anyone agrees that the world would be a better place if no one ever had to kill in self-defence. In that sense, killing in self-defence is a necessary evil. But that doesn't mean that a soldier (or, in D&D, a paladin) is doing something evil when s/he kills in self-defence. On standard, non-pacifistic analyses of defensive violence, s/he does something justifiable - the balance of reasons favours killing, because the perpetrator of the threat has forfeited his/her right not to be killed.
In the standard terminology of criminal law, self-defence is a justification, not a mere excuse, for perpetrating violence. There is no evidence at all that D&D departs from this standard approach at all, given how central defensive violence is to the standard tropes of fantasy fiction (and genre fiction more generally).
When have I ever suggested that killing in self defense, or even in combat, is evil. Every example I've given of evil kills have been a character, after calm deliberation, choosing to kill a bound and helpless prisoner.