I understand why you say that, but the situation is more complex than you think.
If left alone, these three may very well cause a war for no good reason, so eliminating them certainly makes sense.
But the problem is that they've already succeeded in sowing the seeds of discord. If they died, there'd be an even bigger mess:
- Either the settlers go to war because they believe the Lnu killed three of them.
- Or Thorfinn and his supporters get exiled because they enforced their opinion through murder.
There's always the option of making it look like an accident, but then Hild would need to find an animal dangerous enough to plausibly kill 3 armed and experienced men (like a bear), but that wouldn't stop the settlers from still suspecting the Lnu, and they'd still argue they need weapons and walls, if only to stay safe from dangerous wildlife.
The situation is already nearly beyond repair. The way I see it, the only person who could possibly fix this is Ivar. All he'd need to do is tell the settlers that they were wrong, the Lnu never wanted conflict, and that there's no need for weapons or a wall. The only other person who could fix this situation is Styrk, but he likely won't do it, since that would undermine everything he's worked towards so far.