I have to agree that Anders actually seems to be a pretty nice guy, and if Karlach stays in her (in my view) ugly design, that holy Paladin of Tyr will always can count me in for some tiefling slaying.
lol, "holy Paladin", that is no more,
maybe high CHA needed to make it explain,
but if you talk to dead after accepting the kill quest, and then instead killing you talk to her, then return back and confront him, Mr. Greatsword will explain all these