Some of the arguments against altruism claim that people only do altruistic acts to curry favor, signal their superiority, or make themselves feel better.
But doesn't the existence of cranky altruists discount these arguments? By cranky altruism, I mean people who do things for other people but clearly are not happy about it, as in, "yeah, I'll fix your computer, but don't be an idiot and mess it up again because I won't be there to save you." Think of Han Solo in Star Wars, or any reluctant hero.
If they were performing the act solely to curry favor, then I would expect that they would be kind about it, not cranky. If you want to be liked, most anybody can tell you that being cranky is not the way to get there.
If the cranky altruist wanted to signal their superiority, then they would probably pretend that the task were easy for them, instead of complaining about the necessity of doing it.
And if they were doing the altruistic deed solely to feel good about themselves, then wouldn't you expect them to be in a good mood while they were completing the task? Otherwise, you have to presuppose that they are being altruistic to fulfill their needs in some odd guilt model, which I question because I'm not sure that humans are able to predict their future emotions very well.
I'll take a cranky altruist over an overly affable one any day; they're much more fun.