You seem to be raising a couple of puzzles here. One is how it has come about that human beings are sometimes motivated to help strangers, given that we might have expected evolution to produce beings concerned to promote the survival only of themselves and their kin. One immediate answer to this question is that, despite what some fanatical sociobiologists say, you can't explain everything about human behaviour as it is now merely by reference to our evolutionary history -- even if you allow in cultural as well as biological evolution. Evolution made it possible for impartial benevolence to develop, but it didn't necessitate it. Its emergence -- where it has emerged -- is to be understood primarily in terms of history rather than biology.

The second puzzle is why we *should* help others even when doing so doesn't benefit us. Many people have believed egoism -- the view that the only reason we have to do anything is grounded in the promotion of our own well-being. Philosophy has so far failed to provide a knock-down argument against egoism. But what it has clarified is some of its implications -- for example, that the fact that I can save you from years of agony by pushing some button gives me no reason to push that button.