Well, the thing is that they didn't live where we live. And, while claims of armed incursion and the rest might be appropriate, you twisted facts a litt... a lot where it comes to what the elves were trying to do. They were not trying to kidnap a random elf emigrant. They tried to capture a deserter in possession of sensitive information.
Cultural relativism only applies if we care about the standards of the culture being discussed. The Dales elves may not see much wrong with murderous xenophobia, racism, and racial vendettas leading to a massacre- but I am not a Dales elf. I am a Westerner talking with other Westerns talking about if the Dales should or should not be held accountable by a standard of responsibility that is, unsurprisingly, Western in rhetoric and outlook.
I see no compelling reason to in the context of our discussion to defer to the murderer's social relativism, any more than I would defer to the Chantry's justifications for cultural erasure or Tevinter's justifications of slavery or the Qunari's hard-on for order.
Obviously, any villagers they killed after the girl were also their responsibility. Also obviously, the responsibility would be heavier if these villagers didn't attack them. It's stupid if anyone denies the first part. Equally stupid if anyone denies the second. I mean, you actually claim that a deliberate massacre is equal to killing a mob that tried to lynch you? Sure, if you actually did the thing they were trying to kill you for, you're responsible for the situation as a whole. But that still doesn't erase the distinction. Especially if you try and bring our-world modern standards into this.
I don't need the deny to second to deem it irrelevant- the 'mob' in this case is the unjustly attacked party, while the 'lynch' victim is actually the perpetrator.
It's a weird world of victim-blaming, in our-world modern standards or not, that places the perpetrators as the aggrieved party and the victims as the more racist party.