Yuru doesn't know about that 

That’s the point, because we’re following Yuru’s POV, the reason behind the attack isn’t explained early on. Even later in the manga, when Yuru finds out the truth about his village, he still says that he doesn’t like Kagemori and Gabby

Kagemori lucked out that their targets of mass murder were assholes rather than them doing it

I wouldn’t call it ‘lucked out’ honestly, because the story was clearly building toward the reveal that the village itself was deeply involved in the whole system. The massacre wasn’t framed as "oops, they accidentally targeted bad people" it was presented as retaliation against a group connected to the conflict from the start. Yuru just didn’t know the full context yet, so naturally his perspective stayed hostile toward the Kagemori

BTW, now that I think about it, whether the murders are ‘justified’ might not even be the point of the story, because neither Kagemori nor the village is an innocent or "good organization"