[X] Massively reinforce position (-5 Military, founds new March, war with Highlanders)
War happens all the time, you aren't necessarily evil because you go to war and you aren't necessarily the good guy because you don't go to war. The world is more than black and white.
We're totally in the wrong here, but that does not mean we're evil.
I would argue that we are not being evil by going to war, but rather that we are being unlawful and completely against our core social values. While I might be extrapolating, but by so brazenly going to war it might force changes to our social values (Protective Justice, Cosmopolitan Acceptance, just to name two) to adopt a more "might is right" stance.
I am unsure as to who I should quote but there are many people who are voting for letting the HK exact vengeance, and there are many who are (rightfully) fearing that this would let the proverbial "iron weapons" cat out of the bag. While the warriors won't likely know the skills needed to forge iron, what I would be more concerned with is the HK discovering that we are able to craft these weapons (i.e. that we HAVE the know-how).
Furthermore, as AN hinted at it's not even completely certain that the HK will be able to defeat these rogue warriors. If they are not defeated, what do we do? By not supporting them in their time of need we're effectively abandoning them, and I don't imagine the warriors will be too happy about that.
As such, I am advocating for the second option:
[] Attempt to capture the errant warriors and then make amends with the Highlanders (-5 Diplomacy, probable war with Highlanders)
While this might not be successful (HK might very well still declare war on us), we'll have burned our martial surplus and we have demonstrated that we are willing to uphold the law, even if it means to harshly crack down on our warriors. Furthermore, we will now have a casus belli against the HK in the sense that, hey, we at least tried to make amends, and it was them that spat on the offer of reconciliation.