Glolls do not cut up bodies. They tear at and eat them, because of the insatiable hunger they are infected with.
Anders' cultists did not create the gnolls. But the gnolls are more organized than ever before because they are following a leader who is infected with a tadpole. Neither Anders or any of his group have tadpoles. The book on the history of gnolls was in the tollhouse, along with a lot of other books, and not among Anders's possessions.
Anders and his people present themselves as paladins/followers of Tyr. Not as cultists of Zariel. It is his dead compatriots who admit they turned from Tyr and sold their souls to Zariel. And the dead cannot lie. Anders certainly acts as if he regrets certain things, and tries to insist they are not killers. But he is a proven liar. He may have regrets over his decisions that brought him to this point, but he is still a servant of Zariel. It's true that his group cleared the gnolls at the tollhouse, but that is not why they were there. They are just using the tollhouse as a base for the search for the wounded Karlach.
Karlach on the other hand says she was a prisoner of Zariel. I don't believe she mentions selling her soul. Anyway, she is a Tiefling, not a devil. And she has a tadpole. She tells you that Anders and his cultists were sent by Zariel to retrieve her. When the tadpoles cause your minds to link - both before and after you kill Anders - the things you see support her story. They do not prove that *everything* she told you was true, but they do support much of what she said.
So you have Anders' followers of Tyr, who you know are liars that are working for Zariel, and no longer in the service of Tyr. And you have Karlach, who's story is at least partly confirmed through the tadpole, and who you have no evidence is lying.
But that doesn't mean I'm telling you that you should make one decision over the other. Maybe you're playing a character who's not able to determine who is lying. There's also the issue of your character's moral compass. Are they good or evil? Or just in it for themselves? That will impact which decision you make.
Also, if you are interested in spoilers for post-EA content:
People have found in the game files that Karlach is unlockable as a companion. My guess is that we won't be able to recruit her until we meet her again in Baldur's Gate. But of course that can only happen if you don't kill her. I'm still not saying that's the way to go, because it's possible that helping Anders may unlock some future benefit from Zariel down the line. It all depends on how you want to play.
