But Seth isn't the one who is at fault.
Nope, he IS at fault for the stuff he did to Isis, and to the humans he killed, while he let himself be controlled by his feelings of rage and betrayal... and he ends up ackowledging it when Isis confronts him about it. Everything he did to her, and to anyone else, while thinking that neither Isis nor anyone else knew anything about what Osiris had done (in other words, while thinking they were mostly innocent regarding what Osiris had done), is completely his responsibility. That's one of the main points the story is trying to make.
Tbh I kinda symphatize with Isis
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Both her husband and son are in love with the same man who destroyed her life lol