figuring out his sin is hard, especially since battler seems pretty forgetful of his past.
however, if we think about it, what else was the significant thing that happened at that time besides battler leaving? or rather, why dont we look at why battler left?
rudolf and kyrie’s marriage? sure, but why dont we also look deeper in how that happened… thats right, ange’s birth.
now look, dont you guys think that ange having red hair is pretty weird? I mean, look at the whole ushiromiya family, it is pretty unique and different, not to mention its not really a natural hair color. blonde, brown and black hair runs in the family. no one else has it… besides battler.
of course though we know beatrice intent was on using wordplay on his birth to deny battler, you cant dismiss it all entirely. that theory could also be still used just in another way.
so this is my theory:
battler was, as beatrice said, a kid that rudolf took. this explains how he is the only one with red haired in the family at first, that is because he wasnt really an ushiromiya.
however, rather than 6 years ago, it was right at the start with asumu’s kid being swapped, and it is still not sure if rudolf did it as a ploy for the inheritance at that time yet and just wanted some kid to raise.
so lets say things were going well on their family… until one time when battler got too drunk and ended up on bed with kyrie. as for how and why kyrie would do it, perhaps she was also drunk or was reminded of rudolf with battler so she let it happen.
anyway, rudoolf found out about it. and because he’s a good father, as piece battler recounted during his test with beatrice, he tried to spare battler from responsibility and would take in the baby or supply kyrie with funds like he did when battler was rebelling. all while this happened, asumu died some time, though I cant remember if it was before or after ange was born. so rudolf took the next step to make things easier and safer, he married kyrie.
then as we saw how events happened, battler got mad at rudolf and rebelled, all while not knowing his sin. as for how this sin could result in tragedy… that part is sill in development.
maybe I guess rudolf got mad at being a cuck so took the chance to ruin battler’s life once he returned to the conference, or perhaps kyrie got mad at know all about the truth and plotted to kill battler… just that the other people got caught in the crossfire.