after thinking about this quote further
"I say that it is lawful to resist him by not doing what he orders and preventing his will from being executed.”
It doesn't speak specifically to whether we should consider him pope, basically Bellarmine is saying to disobey him, both sedevacantists and sspx disobey him...no?
Resistance to evil is a duty, even if the pope is the one commanding it. No one can be made to follow an evil law.
But Bellarmine is taken out of context, or without regard for context. In the very next chapter, he says that it is certain that a heretic cannot be pope. Bellarmine is not talking about resisting a pope who is a heretic and who tries to pass heresy off as Church teaching; he denies that such a person is even pope to begin with.
Whatever St Bellarmine might be talking about in the next chapter is one thing.
In the present quote, he is speaking about a pope who seeks to destroy the Church.
He says that in such a case, the pope need not be obeyed.
Sedes, on the other hand, say such a pope is no pope at all, thereby contradicting St Bellarmine (who clearly admitted the possibility of a pope who could attack the Church).
Or conversely, by explaining away this quote by citing the next chapter, make his writing on the present point frivolous.
Either way, sedes do an injustice to St Bellarmine by seeking to fasten their schism to his namesake (all the while picking and choosing which of his teachings they will adhere to).