Other forms: egoistically
Egoistic people believe we all put our own needs before those of others. If you're sure that self-interest inspires every human action — yes, even rescuing that sweet kitten from a tree — then you, my friend, are egoistic.
Don't confuse egoistic with egotistic, a word which is often, and wrongly, used interchangeably with it. To be egotistic is to have too inflated an opinion of yourself — in other words, it's a form of self-delusion rather than a particular way of looking at the world. You can be egoistic without being egotistic. Got it?