It is a very good question. Personally I believe that all human action is in some way egoistic, yes. But maybe not in the sense that they only care about themselves.

Allow me to clarify what I mean with that. The basis for my reasoning is that all value, including peoples, is relative. For example, my family is very valuable to me but (I would suppose) only marginally so to you, if at all, and vice versa.

Now what does this have to do with egoism? Everything. Most importantly, it means that you are not always the most valuable person to yourself. Many parents would for example willingly sacrifice themselves for their children because they care more about their children lives then their own, and throughout history we see many times that there are things that people are willing to die for, things that they valued far more then their own lives.

This brings us back to your question whether people who volunteer abroad could be doing it for their own sake. It is true that they might be doing it just for the travelling, indeed I know some who do, or maybe to feel that "givers high". But it could also be that they care for these people, that they are valuable to them, more valuable to them then what they might lose by helping them.

This is how I see the world. For me there are no good and bad people, just people who value different things. Make of it what you will.