The all-consuming dumpster fire in the Middle-East is too complex to simplify, but those three words are at the core of everything. Much like how the different iterations of Christianity (catholic, protestant, orthodox etc...) all got along famously in the middle ages, Islam can be divided into 2 streams that go as deep as being Ethno-cultural. It is INSANE how much blood has been spilled between Sunnis and Shiites just in the last 20 years alone. But those are just it's most recent developments, the feud and it's violence goes way, way, waaaaaay back. Likely it will also continue way, waaaay into the future, as the violence feeds itself. Cycle of revenge and all that.
There is also this other little tidbit called the Sykes-Picot agreement , but that's a whole other can of worms. Suffice it to say, it's how long ago the West permanently fucked everything up and it's why they just won't stop hating us. (only the educated ones, mind you. The grunts just get indoctrinated to hate us, so they do.)