It’s not so much customs or religion at this point, just as the Troubles in Ireland weren’t really about religious belief. Rather, the religious identities mark distinct social groups, which have a history of discrimination, oppression, and terror against one another.
It’s not “You don’t believe in my God so I have to kill you” so much as “Your daddy killed my daddy, so my uncle killed your uncle, so now you and I have to try to kill each other too.”