Sedevacantists are Catholic.
So were the Jansenists. But they were still mistaken. Believe me, I can understand loathing much of what Pope Francis does---most of what he does, in fact--- but we can't jump off the ark and abandon the seat of the Papacy, no matter who occupies it. It's a sad day when you have to pray for the Pope's conversion, but Catholics must respect his office, if not the man. That is where sedevacantists make their error. He could be a terrible Pope, but until he dies or abdicates he's still the Pope. Ours is the sort of difficult era that produces the greatest villains and the most heroic saints.