Iirc, the original definition of an antihero didn’t mean a bad guy you root for. It meant a hero in the story that didnt embody the traditional heroic traits of courage, strength, faith in God, noble morality, etc. The antihero is the hero who is not really heroic - a hero who is kind of just a normal person. An example of this being Don Quixote - a man who spent his time fighting dragons and rescuing princesses only in his imagination.

These days, almost all heros outside children’s books are antiheros. Because true heros are flat and one dimensional and unrelatable and don’t contribute to an engaging plot. These days, it isn’t thrilling enough for us to hear how the knight slew the dragon. He must first overcome… I dunno, his childhood trauma or something… And then he can slay the dragon!