If I don't like someone, I'll try to find out why. If that person doesn't deserve it, then I wouldn't have any sympathy. As a kid I got walked on too much without saying anything. Always nice, too good, too stupid. It made me think and today it's out of the question. If I don't like someone and they deserve it, at best I'll send them "shit". I wouldn't be particularly bad, as long as the person doesn't provoke me, it will mostly be as if they don't exist

I don't think that is fair to them. You are free to dislike whomever you want, but using them is where things start to go wrong. Although I think it would also depend on the mechanisms of your relationship overall and the reason behind your dislikeness toward the other person. If it is someone who has done the same to you in your prior friendship or relationship with them then I wouldn't say it'd hurt to get some benefit out as well. 
But at the same time, fooling someone in the long run to continue a 'friendship' or any sort of relationship isn't only toxic to them but to you too.