He already "resigned":
Therefore, the true opinion is the fifth, according to which the Pope who is manifestly a heretic ceases by himself to be Pope and head, in the same way as he ceases to be a Christian and a member of the body of the Church; and for this reason he can be judged and punished by the Church. This is the opinion of all the ancient Fathers, who teach that manifest heretics immediately lose all jurisdiction....
- Saint Robert Bellarmine, Doctor of the Church, De Romano Pontifice, lib. II, cap. 30
I hadn’t ever seen that before. Interesting.
There is the sin of heresy, and there is the canonical crime of heresy. They are distinct.