Child abuse and elderly abuse. Any of the others can fight back, they just don't. A child can also grow up and fight back. Geriatrics, if they are incapacitated, will never be able to fight back. I've seen this before. They are said to have "dementia" and cast aside to die. I care more about elderly abuse.
However, child abuse of the sexual kind is the worst offense. I try to be a good person, but if I ever saw someone sexually abuse a child, I'm quite positive I would kill them.