Based on his surviving writings, St. Patrick certainly held to Catholic views that are not shared by all Protestants, as he baptized and used chrism (consecrated oil) in the Sacrament. He held that the Church’s priests have the power to bind and release, quoting Matthew 16:19. He consecrated monks and nuns in Ireland and ordained priests. St. Patrick quoted scripture directly and made allusions to scriptural passages extensively, including the deuterocanonical books that many Protestants reject as inspired but which Catholics include in their Bible (such as the Book of Tobit, the Book of Sirachus, and the Book of Wisdom). He believed Baptism was essential to salvation. It would be hard to argue that St. Patrick was not Catholic.
In one of the texts he wrote, an exhortation to the soldiers of the pagan king Coroticus, he was clear that he recognized Rome as the seat of the Church, and recognized the hierarchy of his superiors, although he sometimes chafed under it. He stood in opposition to the pagan kings, who enslaved and murdered Christian converts and sold women and children into prostitution (the pagans of Ireland were not a gentle, mystical bunch, but more like what Patrick called “gangsters”)
We can also read that he held that their pagan gods were actually demons (a common view held by the early Church fathers), and it could be supposed that the legend of St. Patrick driving the serpents out of Ireland was symbolic of him dispelling the evils of Celtic paganism - not killing pagans, as it is obvious he possessed little temporal power to do so, and the killings were by the pagans. In Part II, Section 9 of his letter he inveighs against taking life, using scriptural justification, so he was unlikely to turn around and kill others, pagan or not. He wanted to convert, not kill. Every Irish pagan who died without repentance would have been considered a loss to Satan by St, Patrick.
It’s also worth noting that St. Patrick, like St. Augustine, St. Nicholas, and St. Paul, stood boldly against slave trading and sex trafficking, and risked his life in doing so.
You can read his letter, which in context tells you a lot about how feisty the bishop was, here:
ancienttexts.org/library/celtic/ctexts/p02.html
In his Confession (the other surviving work by him), he states clearly that he did no harm to the pagans:
"You all know, and God knows, how I have lived among you since my youth, in true faith and in sincerity of heart. Towards the pagan people too among whom I live, I have lived in good faith, and will continue to do so. God knows that I have not been devious with even one of them, nor do I think of doing so, for the sake of God and his church. I would not want to arouse persecution of them and of all of us; nor would I want that the Lord’s name should be blasphemed on account of me; since it is written: “Woe to the one through whom the name of the Lord is blasphemed.”
You can find a translation of his Confession here:
confessio.ie/etexts/confessio_english#