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I think it is great that you, one of my Protestant brothers/sisters in Christ, has taken an interest in past Popes.
I checked the “Papal Bull” list for these two popes and it seems they were not very prolific. I reviewed both of these encyclicals and I do not see anything in them that is against Christ.
Pope Alexander VI - The Inter Caetera (relates to borders and Spain)
Pope Nicholas III - Exiit qui seminat (relates to rules for Friars)

You see showing a Catholic that a Pope was a sinner is useless if you want to convince a well-chatechized Catholic to convert to protestantism. You need to show that the papacy made an infallible doctrine that was somehow wrong, and prove that. Now most encyclicals, or “bulls”, are not infallible. The two mentioned above are not infallible. That being said, you will also find that the non-infallible encyclicals issued by Popes are generally wonderful documents, regardless of the author’s sinfulness.

The Nicene Creed is a core infallible declaration by an ecumenical council of bishops and the Pope - see what you don’t like in that.