To be fair, it wouldn't be the first time that we'd recruited from a previously thought unflippable enemy. For my part, I have different standards for mortals and outsiders because of differences in psychology and relative control over their own nature.
For mortals in cases like this, one of my evaluation points is how abstract the consequences of what they do are from their perspective. For a mercenary killing people for a reward isn't an unusual idea, so doing it on an alter might not seem so bad; especially if they don't really understand the consequences for the victim beyond death. Even when they do know, it's out of sight, which makes it easy to ignore.
None of this excuses anything, but to me there is a difference between doing this kind of thing with a visceral understanding and an superficial one. We have actual fiends on payroll that are less personally monstrous (and easier to reliably work with) than a mortal who has the capacity to be different and instead chose to cosplay as a demon.
I guess I was just catching up to the rest of the thread on this point, since I thought at least some of them would be regular asshole mortals and not monsters that haven't had a chance to rent an apartment in the lower planes yet.
