Responses
Yep. As you say, we do know why he killed already (it was explained in chapters 36 and 39 and others): He needed to kill innocent humans and destroy their souls in order to use their feelings of hatred and betrayal to create a cursed weapon that would work against an undead like Osiris, as he knew his ordinary weapons wouldn't be capable of doing a thing to an undead god, only a powerful cursed weapon would be effective against an undead like Osiris.
Some of you will hate me for this.
I feel bad for Seth but I don't think it's enough reason to forgive him for his crimes. Past traumas are not an excuse to create hurt for others. They can be used as a way to understand certain actions but should never be an excuse for them.
I know this is just fiction but when I feel bad for him I think would I feel the same way if I saw him the way those women he'd killed did? It's because he's a main character that we can see him in more depth but because of that I forget that he also did a lot of 'bad things' too.