We really don't do anything without it making us "feel good" on some fundamental level. Motivation/intrinsic reward for behavior goes to the very core of our being as self-directed organisms. So asking "how is donating any less selfish than not donating" is asking the wrong question. We have evolved to take pleasure in behaviors that are beneficial to ourselves. One who is selfish will take pleasure in things that are solely beneficial to him or herself. One who is unselfish has a reward system that is aligned with the benefit of others. So when we say someone is a "good person" or "unselfish", we are really judging the alignment of their reward system as being more suitable for a social environment.

As an aside, note that this way of thinking is compatible with determinism: while the murderer may not have any more control over his anti-social behavior than the altruist, the person is condemned for having a brain wired in such a way that is toxic to a collectivist society.