Apparently sibling rivalry is as old as siblings themselves. Consider Cain and Abel- they only had each other (although the Midrash indicates they also had sisters), yet look how they fought.
You have to wonder how Cain, reared by parents who spoke to G-d Himself (in fact, they chatted just before he knocked his brother off) went so extremely off the rails- and murdered his own brother!
This may just have been the first of thousands of conflicts that were sparked by religion. After all, the Cain-Abel fallout started as a religious exercise, each bringing an offering to G-d. They experienced the typical “my-way-to-G-d-is-better-than-your-way”. Only, in this case, G-d took sides.
Cain offered a simple grain-offering. Abel sacrificed a prized animal. G-d accepted Abel’s offering and turned away from Cain.
Abel was furious- not with G-d, but with his brother. Instead of contemplating why G-d had ignored him, he shifted the blame to Abel.
Cain didn’t bring an offering to serve G-d, but to satisfy himself. Cain wanted to get away with the bare minimum to satisfy himself that he had serviced G-d. Abel happily stretched himself beyond his means to satisfy G-d.
“Commitment to G-d” that’s based on the what’s-in-it-for-me philosophy can have devastating results. Religion and spirituality are about Higher Purpose, not Self.
If our world had more Abels and fewer Cains, we’d be living in peace.