I can't tell you accurately, but the whole mess started bc Osiris is obsessed with Seth. It's so severe that he made Seth infertile and impregnated Nephtys himself. Osiris wants Seth for himself, he also raped him (by blackmailing him with Anubis) and Isis saw that and misunderstood that they betrayed her and she was only blaming Seth and thats how their conflict became so serious.
It looks like Seth is a god of war but a good one, who protects all of Egypt and was well respected, like Horus told him how much he admired him.
I assume that Seth became mentally unstable bc of Osiris and made terrible wrong choices to cope with everything.
I hope this makes a little bit sense (￣∇￣")
Btw: Osiris, Isis, Seth and Nephtys are Siblings and are children of Geb (Earth) and Nut (Sky)
Help ! I swear there was a panel where someone said something like " Osiris didn't lust for Nephthys but for Seth" and there was a flashback where Osiris is looking back at Seth. I thought it was Sekhmet who said it but i reread it all and didnt find it.If you know pls tell me.