I'd try my very best to convince them to go back to the light, with all my might. If they're too far gone to the point of committing horrible crimes, and their soul has been entirely lost, then I'll go on a quest to try and prevent them from doing more harm, trying to fix their shit, and reluctantly kill them with my own hands, or die trying. And then be incredibly sad about it for the rest of my life. But at the end of the day, my morals & principles are always gonna trample any love I can have.
And yes, this is what I'm gonna do with Solas in Dragon Age 4. I hope they'll write several ways to deal with him though, it'll really break my heard in a million pieces if I have to kill him. I'll do it, but... fuck, at what cost.