Oooooh, RBarbare --- I love your train of thought! The part about dismembered limbs is fascinating, but that could have been the handiwork of gnolls too; we see dismembered cadavers anywhere they hit. Anders & the Zariellites could be keeping the bits and limbs out in the open to attract more gnolls to kill, pass the time, etc.
I don't think that Anders is a reluctant servant of Zariel -- au contraire, I think that after the fall of Elturel and what he and his companions went through he would serve her even if she didn't have a hold on his soul, but that's not how archdevils work. He might just have a bit more wiggle room to focus on gnoll-fighting and innocent-protecting instead of hunting one runaway tiefling.
There's also a meta argument for keeping both factions alive; they seem perfect candidates to become part of your party. The Zarielites have the same composition as the Origin companions: fighter, rogue, healer and mage, which suggests that they might become a mercenary group for solo players at some point. Their structure mirrors the "adventurer / grave robber" group that you encounter at the very beginning, in the overgrown ruins, and even Aradin's band back at the Druid Grove. Plus there's the whole "don't kill the merchants please" capitalistic reason to keep Cyrell alive.
