How do you let someone know that you care about them but they'll be better off without you anyways.

You don't. You either man up and be what they need to be or you leave and accept that they are going to be hurt and might be angry or resent you. But if you truly care about them you live with that resentment towards you and let them go. Maybe they will, maybe they won't resent you but you gotta accept it instead of trying to make your standing better and yourself better. That's part of caring. Accepting what's best for them, and not just your ego.