Do you think that Paul would mistreat somebody because they were - for example - pagan?
You mean like doing things like calling themselves a believer and sleeping with his father's wife in the name of Christian freedom? Yeah, I think he might 'mistreat' someone like that (assuming that we accept public confrontation and excommunication as 'mistreat'.)
I think Paul was far more likely to 'mistreat' false teachers. like the Judiaisers that he suggested that rather than circumcising gentiles, they should just go and emasculate themselves!
He also had a tendency to 'mistreat' brothers in Christ who he saw as unreliable or hypocrites, like his public confrontation of Peter's hypocrisy with Jewish and Gentile Christians, and his fight with Barnabas over who to bring on the second journey (although these may have been character flaws, since Paul was not Jesus and, therefore Paul was imperfect).
Now to the question, would Paul mistreat an unsaved pagan because they were a pagan, no.
1 Corinthians 9:19-23 [NKJV]
For though I am free from all men, I have made myself a servant to all, that I might win the more; and to the Jews I became as a Jew, that I might win Jews; to those who are under the law, as under the law, that I might win those who are under the law; to those who are without law, as without law (not being without law toward God, but under law toward Christ), that I might win those who are without law; to the weak I became as weak, that I might win the weak. I have become all things to all men, that I might by all means save some. Now this I do for the gospel’s sake, that I may be partaker of it with you.
