I'd encourage you to focus not on "what REALLY happened to St. Barbara," but rather just the movie. It is a movie about love, a mother's, a friend's, and eventually, about God's love. You don't see any Christian evangelizing done, in fact you may wonder at the end why Barbara even became a Christian; no doctrine was preached to her. But it's not that kind of movie. If anything, it presents what was really happening back then, and what was a noticeable thing about Christians, during that period of wars and terror and man's abuse of his fellow man. The "strange," noticeable thing about Christians, as quoted by Roman historians, was: "See how they love one another." This movie showed how, initially, it was Christian love and sacrifice which made Barbara a convert.
In the movie's presentation of Roman citizens, I saw much of our society today. There were good people, there were laws intended to be good, and there was a state which said its laws, ANY laws, were supreme over any religion. And many good people couldn't understand why Christians wouldn't obey those laws, "made for their good." Many still don't understand today.