On Not Killing "Monsters"

Tonight, my D&D character let some traditionally "monstrous" and "evil" creatures live. One of them was a lizardfolk who made no attempt to harm us, and the other was a necromancer who only cast a harmful spell to help him cover his escape when he was attacked. Neither of them was observed doing anything evil, but both admitted to serving a black dragon, which are almost always evil. Granted, the same can be said of normal lizardfolk are necromancers. Lizardfolk are typically evil because they eat people, and necromancers are typically evil because they use dark magic and undeath. Yet, I saw no evidence that either of these "monstrous" individuals had ever harmed anyone. So, my character chose not to harm them.

Granted, this may come back to bite them. We will encounter the black dragon at some point, and when we do, that conversation may not end well. If we end up fighting the black dragon, we may end up fighting the dragon's allies. The necromancer, at least, is probably going to show up again, probably as an attacker, probably with some undead allies. But I suppose we'll have to cross that bridge when we get there. So far, my character has judge people by their actions, not their race or profession, and I prefer it that way. My character will fight anyone who attacks her, her allies, or any other innocent person, but if they're leaving others alone, she'll likely leave them alone, too. Unless people cause harm, they shouldn't receive harm, either, no matter who or what they are.