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.
1 Timothy 3:1-13 1 Timothy 3 1The saying is trustworthy: If anyone aspires to the office of overseer, he desires a noble task. 2Therefore an overseer must be above reproach, the husband of one wife, sober-minded, self-controlled, respectable, hospitable, able to teach, 3not a drunkard, not violent but gentle, not quarrelsome, not a lover of money. 4He must manage his own household well, with all dignity keeping his children submissive, 5for if someone does not know how to manage his own household, how will he care for God’s church? 6He must not be a recent convert, or he may become puffed up with conceit and fall into the condemnation of the devil. 7Moreover, he must be well thought of by outsiders, so that he may not fall into disgrace, into a snare of the devil.
8 Deacons likewise must be dignified, not double-tongued, not addicted to much wine, not greedy for dishonest gain. 9They must hold the mystery of the faith with a clear conscience. 10And let them also be tested first; then let them serve as deacons if they prove themselves blameless. 11 Their wives likewise must be dignified, not slanderers, but sober-minded, faithful in all things. 12Let deacons each be the husband of one wife, managing their children and their own households well. 13For those who serve well as deacons gain a good standing for themselves and also great confidence in the faith that is in Christ Jesus.
Titus 1:5-16 5 This is why I left you in Crete, so that you might put what remained into order, and appoint elders in every town as I directed you— 6 if anyone is above reproach, the husband of one wife, and his children are believers and not open to the charge of debauchery or insubordination. 7For an overseer, as God’s steward, must be above reproach. He must not be arrogant or quick-tempered or a drunkard or violent or greedy for gain, 8but hospitable, a lover of good, self-controlled, upright, holy, and disciplined. 9He must hold firm to the trustworthy word as taught, so that he may be able to give instruction in sound doctrine and also to rebuke those who contradict it.
10For there are many who are insubordinate, empty talkers and deceivers, especially those of the circumcision party. 11They must be silenced, since they are upsetting whole families by teaching for shameful gain what they ought not to teach. 12 One of the Cretans, a prophet of their own, said, "Cretans are always liars, evil beasts, lazy gluttons." 13This testimony is true. Therefore rebuke them sharply, that they may be sound in the faith, 14 not devoting themselves to Jewish myths and the commands of people who turn away from the truth. 15 To the pure, all things are pure, but to the defiled and unbelieving, nothing is pure; but both their minds and their consciences are defiled. 16 They profess to know God, but they deny him by their works. They are detestable, disobedient, unfit for any good work.