Usage patterns
〈人〉は利己主義だsomeone is egoistic/selfish
He is often criticized for being egoistic.
利己主義に走るto give in to selfishness
During the crisis, people gave in to selfishness.
利己主義的な〈事〉egoistic (way of thinking, behavior, etc.)
His egoistic way of thinking became a problem.