Very good point that I think clarified this for me.

Per Wikipedia, "Altruism or selflessness is the principle or practice of concern for the welfare of others." That seems like a plausible definition, and I think it illustrates what's wrong with this whole chain. The issue here is not increasing concern or practice but expanding the definition of "others"; that is, bringing more people/animals/objects into the realm of concern. So if we taboo altruism, the question becomes to whom/what and to what degree should we practice concern. Furthermore, on what grounds should we do this?

For instance, if the real principle is to increase pleasure and avoid pain, then we should have concern for humans and higher animals, but not care about viruses, plants, or rocks. (I'm not saying that's the right fundamental principle; just an example that makes it clearer where to draw the line.)

In other words, altruism is not a good in itself. It needs a grounding in something else. If the grounding principle were something like "Increase the status and success of my tribe", then altruistic behavior could be very negative for other tribes.