I'd lost track of this, but here's the thing. Killing the raiders is more likely to draw the attention and anger of other Lnu tribesfolk, rather than stop the escalation. Already there are groups who are convinced Thorfinn and his people are dangerous. Proving them right by killing them isn't going to help; it's all about perspective. Driving them off, rather than killing them, shows strength, and shows that no, you aren't dangerous, but rather are defending yourself. Unfortunately, though, it looks like their situation has already escalated to the point that it's more a question of if Thorfinn and his group can leave without the Lnu pursuing out of vengeance. That's the more significant long-term danger.

STOPPING them is a good thing, of course. KILLING them, however, is likely to only confirm the thing the Lnu fear most: that these people are a deadly danger. Ironically, Thorfinn's goal of making them leave them alone long-term is actually better handled by driving them off without killing them. This leaves him and his as the strong-but-safe people who defended themselves, and the group who attacked as foolish people who killed for greed and then were repelled when Thorfinn and his people clearly had great strength if they so chose.

It builds the narrative for Thorfinn that he is not, in fact, a threat to the Lnu. It looks like the story is going to involve them leaving. One thing Thorfinn does not want is pursuit. If they come for payback or believing Thorfinn and his folk are too dangerous to let settle somewhere else, Thorfinn will be quite well served in having taken no lives, leaving them as the aggressors in their own peoples' eyes.