rambley perc'ildan thoughts below
the hate sex between vax & percy not being 100% hate sex because they trust each other so much and really do have a strong bond that it would be unfair to say that they hate each other. (really, at best they just butt heads once in a while and disagree on some matters, but they still lean on each other in the end)
but there's still some sort of hate there, but it's not hatred aimed at the other, it's hatred aimed at themselves because percy and vax both hate themselves so much, but they trust each other so much
so their sex is spiteful and angry and vicious and cruel, but it's not to the other person, (never the other person), it's to them because they hate the face they see when they wake up in the mirror.
and no one else really gets that like they do, because it's not healthy, but it helps, and they can trust each other to not say anything, not judge them, just take the brunt of the words and the actions. it's just fucked up and grounding, and they need each other to help them because it's somehow healing and it helps them.
(and even in the end, there's always that soft moment where they ask if they're okay, if they need something, to reaffirm that "I love you for you, scars, flaws, and all, and I hate that you don't see the man I do and I would give my life, heart, and soul for you to.")