When I speak to Anders I'm able to confront him about not actually being a Paladin of Tyr to which he claims they're not murderers after the Halfling says to lock the doors. They also are friendly even if you exhaust most dialogue options other than the obvious ones that say you're going to kill them.
To sum it up Anders says when he prayed to Tyr when his family was slaughtered he didn't answer, but Zariel did so he, also everyone there, had made deals with Zariel and need to retrieve Karlach. They need to retrieve her because she is an expert demon slayer and 'champion' of Zariel/The Blood War.
If you kill Anders and crew Karlach will tell you her plans to slaughter the ones who ratted her location out when she gets to Baldur's Gate and to avoid it when she gets there. I think she's going to be the Bard companion?
