If you look at Judaism, Christianity and Islam, you notice they are constructed in a way that it is difficult to reconcile them if you take at least one of them as true. For example, the most holy day in Judaism is Saturday, in Christianity it's Sunday, in Islam it's Friday; this alone would make some people want to kill the other groups. Then Judaism doesn't accept Christianity because it doesn't adhere to what is expected from Messiah, perfect justice and tradition. Christians on the other hand blame Jews for killing Christ, a Jew, and the main actor in this, pharisees, is what became the prevailing type of Judaism, causing disdain. Islam accuses both Judaism and Christianity of heresy, usurps Jerusalem as exclusively its holy site, and the newer Quran suras, which are more militant to other Abrahamic religions, supersede the older, milder ones (which is how Islam deals with internal inconsistency). What you can notice is that all three religions use each other as scapegoats whenever something goes wrong.

Imagine we live in a simulation, or a real-time strategy game of beings we can't comprehend; how would you make sure there is a permanent conflict in your game? Just make a few similar but incompatible religions claiming they are the right ones and the others are corrupt and watch the fun...