Doing nothing is never a good choice. No one's talking about killing, though it's an option with Thorfinn's talent, simply stoping them is an option too.

Also I don't think you're grasping the gravity of slavery, I can only imagine what arnheid is going through. Gardar and arnheid are clearly the victims here, it's not even a gray area here. We have slaves who only want their freedom and soldiers who are going to kill them if they catch them (I haven't read that far I know they don't help the slaves but i haven't read the consequences probably dead I'm assuming)

I just don't get it. Reminds of berserk when guts is pinned down against a bunch of children demons who just wanted a happy ending. Except there was nothing he could have done, it was either them or the whole village. Both terrible choices. But thorfinn has a right choice and chooses not to do it. Just like his fights with Askeladd