"If they're unrelated to Islington then it makes no difference. Just like we wouldn't defend humans we aren't aligned with that cause problems, I doubt they'll involve themselves just because it's their kind.

If you're uncomfortable with killing then you don't need to be there." It's much less that Sasuke 'won't mind' killing and more that he's looking forward to both the revenge aspect and the ability to protect those he actually does care about by wiping out this particular existence.