The source for the first part on Barabbas is the Bible, where it specifically states what he was arrested for, which was treason to Rome.
The source for the part about him not existing is just searching Barabbas on Google. While there is plenty of evidence suggesting that Jesus, as a person, existed, there is no evidence to suggest that Barabbas was a real person.
Also, the whole thing about Roman slavery being less cruel is both correct and incorrect. The thing about Roman slavery was that there was as much a caste system in slaves as there was in the free populace. This is also without including indentured servants, so keep that in mind. But slaves in Rome could be treated very well, by the standards of slavery, or very badly, depending on where they ended up being slaves and who they ended up being slaves to.
The main reason that Roman slavery is looked on as a much less harsh version than other types is that Romans allowed conquered, slave nations to continue to hold onto their cultures and religions, whereas lots of other groups would not allow that.
But, as I was saying, it all depended on where you ended up as a slave. Such a thing was also mirrored in the American slavery system with some slave owners, particularly ones who inherited slaves, treated them better than others, but in Rome it was much more pronounced.
The subject of Roman slavery could fill several textbooks alone.