I think in any scenario, even if they're a vile parent, it's wrong to CTB without at the very least ensuring they're cared for somehow. No one deserves to have that sort of event traumatise them, because it would to almost every person on Earth. I wouldn't wish that to a soul on Earth.
If you do decide to CTB, please make sure there's a way for them to be taken care of, by a carer or care home or something similar, or even just friends...it's not fair on them, no matter what they've done to you in the past. No point hurting them, otherwise your suicide isn't "freeing", it's more just revenge at that point, and that makes it seem as if you just want to reject whatever your parents want, not to actually die.