First of all, what do you define evil as?

Let's take people who do good acts to make themselves feel better, i.e. to boost their confidence or self-value. What makes that evil? If this act benefits everyone maximally, isn't it actually the highest good (since it hurts no one)?

Why must complete altruism be the only good?

Can complete altruism even be achieved? Is there any virtuous act that I can commit without affecting myself in a good or bad way? Can I commit an act that benefits others while not affecting myself?

Going back to people who act in terms of non-materialistic confidence or self-value. Someone who volunteers because helping others makes them feel happy. Assuming that they were given your mask (or the Ring of Gyges), wouldn't they continue to commit acts that help others?

On a sidenote, I would recommend reading on Epicurus. He shared your belief that humans are hedonists, but challenges you in your ethical conclusions from that belief.