The big issues are justification by faith alone and imputed righteousness, in terms of salvation. Roman Catholicism rejects justification by faith alone and imputed righteousness. These are core salvation teachings.
Sola Scriptura is another gigantic issue. Evangelicals believe Scripture is the reliable guide to salvation. Roman Catholics believe that Scripture and the Church is the reliable guide to salvation, but that the Church is the infallible interpreter of Scripture. Therefore, a true Roman Catholic would defer to the Church in their understanding, even if the Church's teaching is counter biblical.
The Five Solas are the best way to compare Roman Catholicism with evangelical Christianity. The Solas were meant to point out issues with Roman Catholicism.
Sola Scriptura (Scripture alone) vs. Scripture and Church councils, papal bulls, traditions
Solus Christus (Christ alone) vs. mediation of Christ, Mary, the Saints and the Church, works
Sola Fide (faith alone) vs. faith plus the sacraments and works
Sola Gratia (grace alone) vs. baptism and other meritorious human works
Soli Deo Gloria (God's glory alone) vs. human cooperation, veneration of Saints and Mary
However, I would criticize evangelical Christians, in some ways, of being practical Roman Catholic in their beliefs. In some sectors, they are even worse than Roman Catholics. They may believe that justification is by faith alone, for example, but they do not couple that with an understanding of union with Christ, which produces obedience through the indweling presence of Christ within the believer. Examples of this include some Independent Fundamentalist Baptists, Free Grace Movement, and churches associated with Grace Evangelical Society.
As far as whether an individual Roman Catholic is saved or not, it is possible, especially if they don't understand the Roman Catholic official teaching. I was once a cultist and I was ignorant about the teaching of justification by faith alone and imputed righteousness. I was still saved, but ignorant. I think it is possible to be saved, but ignorant. If the Roman Catholic knowingly teaches against justification by faith and imputed righteousness, though, he has rejected a core Christian truth.
I read the cult founders' books after many years, and I do not think he was saved. He knew the evangelical Christian position regarding justification by faith alone in Jesus Christ alone, but he rejected it knowingly. Therefore he depended on his own righteousness, and I believe he was lost. If he knew the concept of imputed righteousness, and knowingly rejected it, he is in no better situation than the Israelite who Paul said was forsaken in Romans 11.
It is funny how different people I respect view whether a Roman Catholic could be saved or not. One educated person said he thought the little old lady in Mexico who had a simple faith but belonged to the Roman Catholic church may be saved. Another educated person said that he thought the Roman Catholic believer who knew more theologically may be saved, because he knew more of the "truth" than the little old lady in Mexico, although he still belonged to the Roman Catholic church officially.
I am by no means sympathetic with Rome but consider it an apostate church. However, there are many evangelical churches that I consider to be in pretty bad shape, including ones that ordain gay men and women or approve of homosexuality or abortions. And, a lot of the health, wealth and prosperity churches have horrible doctrine, too.
Additionally, there are evangelical believers (although I wouldn't call them true evangelicals) who don't have the first clue about the Five Solas and their importance to us today. So, how can I fault a Roman Catholic when an evangelical may be in exactly the same position as him, practically speaking, except without the smells and bells? We have way too many ignorant Christians due to the Revivalist movement, and personal laziness.
Here's a list of good descriptions about the five solas:
https://www.gotquestions.org/five-solas.html
https://www.gotquestions.org/sola-scriptura.html
https://www.gotquestions.org/sola-gratia.html
https://www.gotquestions.org/sola-fide.html
https://www.gotquestions.org/solo-Christo.html
https://www.gotquestions.org/sola-gratia.html
For someone who wants more information, I would recommend this book:
https://smile.amazon.com/dp/0875526705/ref=cm_sw_em_r_mt_dp_U_CRIHEb4D3DNGG
Regarding how we should treat one another, I don't see much value in arguing with them. I am pretty patient with Roman Catholics. I am talking with one guy who is Roman Catholic on a regular basis, and he and I have been friendly in our conversations. But, then, he knows I don't believe a lot of the ignorant Dave Hunt, Jack Chick-level propaganda about Roman Catholicism, which may or may not be true.
It discredits Christians if they believe ignorant propagandists like Dave Hunt and Jack Chick.