So sometimes I see arguments that Gabe was a bad person, but killing him was wrong.
I'll explain why I think it was right.
First off, Sally only stayed with Gabe because his scent warded off monsters. She endured not just verbal, but physical abuse as well.
And she couldn't just "let Percy go off to CHB and train before she ever met Gabe."
Poseidon probably did tell her about the prophecy at one point, and she knew it would almost be a death sentence, especially at such a young age. And if Percy knew sooner, then he would've been found sooner.
Now, Percy is learning how to fight monsters, he's old enough to understand, and something is happening so the gods CAN'T outright kill him without messing things up (unless Thalia, Nico, or Bianca became a child of the Prophecy. Or Jason would enter sooner). Now it's the right time to send him off to learn how to fight.
And she couldn't just leave Gabe. He's not past hitting her. He could've even killed her if he wanted. If she filed for divorce, he could've gone after her. But even then, she is poor. She doesn't exactly have the money to hire a good divorce lawyer. And Gabe could've gotten rights for some custody or visitation with Percy. We know how poor people are treated when it comes to these situations. Especially during the time it was happening.
And just running off in the night wouldn't do right either. Gabe wouldn't be happy with that. He enjoys overpowering and control.
Her disappearing would make him angrier. He would break the news out about how "he dearly misses his wife and step-son."
Not only that, but it was either take Percy with her, which would cause them to sooner be seen on camera's because monsters would follow them because Percy, or he stays at CHB for forever which would make it seem like he'd gone missing the SAME TIME Sally ran away. Which would naturally be seen as very fishy, especially since her son is a known trouble maker.
And again, they aren't rich. What could she do against him?
Her whole reason to staying with him was to keep monsters away from her son. But now that Percy goes to CHB in the summer, and his existence is known now, that won't do.
She couldn't have just run off. Gabe would have her found.
She's poor, and Gabe kind of does have some rights over Percy, and some step-parents can get visitation rights after a divorce.
Not only that, but going on the run would have to keep Percy at CHB all the time. Destruction would follow, now that the prophecy is closer, he's a son of a Big Three god, and camera's would see them in stores, on the street, etc.
Her only option was murder. Both for her son, and for herself.
She narrowed down her options, and came to the only option she could: killing.
Not only that, but pretty sure she also killed his friends too. They were enablers. They were fine, and happy, with her being abused. Could have even hit her themselves. This was a kids book, so we don't really know what she endured. A kids book can't exactly go into talking about how a step-father let his friends hit and talk down to his wife. And she probably never let Percy see that side.
And if they acted like this so openly with each other, who knows what some of the others were doing with whoever they lived with. They could've been abusing their own spouse and/or kid(s) at home.
So what Sally did was literally her only option, and may have helped other victims too.
So Gabe being murdered was the only decision, unless the books took a whole different turn with not just Percy's Half-Blood life, but also with him worrying about his mother and maybe even accidentally bumping into her at some point(s).
