I can think of no group of people who categorically are above being killed. I personally don't kill because I have no interest in it, but, hypothetically- Lets play this out as if I were a super-sadist serial killer or something:
There are a number of people I would not kill out of self-preservation. High-profile people like celebrities or politicians, police etc- people whose death would cause greater response. People close to me would also make me a suspect too soon for comfort.
I like the idea of honor, and the idea of myself as being 'honorable' (I'm often not, but I like the idea and I make an effort when it is not too much trouble or a hindrance to something I want), so I would be unlikely to kill anyone who I didn't see as deserving in some manner. I don't think anyone is too innocent, or good, or whatever to die- murder is cutting life short and seems unfair, but so does cancer. Fairness is a mythical concept when held to reality and nature. Unsporting would definitely be the word for people who are incapacitated in some fashion or another- but if (for some reason, lets say I'm an assassin, lol.) they had to die, I would likely see their vulnerability as a simple advantage- making the killing less effort, or cleaner. Killing people close to me would also have the side-effect of being both boring and disadvantageous. I know the people near me, and so their reactions, I also depend on them to be in my life as the few examples of this species that I don't have the urge to see caged.
But as for any reason besides self-preservation or lack of interest- no, not really.