Paladin and rogue in the same group - what's allowed and what's not?

[The Random NPC]

In that case, the only justification I can think of is that since they are made of pure evil, killing them would remove it from the multiverse.

Sidenote: I do agree that a morally ambiguous devil is the better way to go, and that any paladin that kills them with no better reason than their race should fall.