Even before the rise of Monotheism, there have been conflicts between people of different faiths.
But especially in the last few thousand years with the rise of Judaism, Islam and Christianity the ante got upped.
If you already believed in Zeus, Hera and a host of other gods up on mount Olympus, and some stranger came along talking about some other god, it was easy enough to just mentally add him to your already-large pantheon of belief.
Monotheism created the idea of "one true god," which suddenly made all those other gods false.
Today there are many like you Jae, from all faiths, who look at any and all religion, and see them all as just different names and faces on the same entity, kind of an 'all roads eventually converge' kind of religious view.
Of course there are factions from virtually all faiths who, sometimes violently, do not accept other faiths whatsoever.
I don't really have an answer, just wanted to share my background view, and say that it'd be a better world if more people didn't mingle their hate with their religion.