Was there a text as revolutionary regarding humanitarism as the New Testament in a pre-Christian era?

Actually, I'd say there's a pretty good chance you are misinterpreting some of these things. For instance, "Turn the other cheek" is often misinterpreted by modern people. It really should be known as "Turn the left cheek". It isn't about "humanitarianism" at all, but rather dealing with an oppressor. See the accepted answer for When do we as Christians draw the line on self-defense? on Christianity.SE.

As for your general question, The Beatitudes I think are fairly instructive here. To Christians (such as myself) this is arguably centerpiece of Jesus's teachings. However, most of the concepts in the Beatitudes can be found in the Old Testament, so the ideas themselves had clearly been around in Semitic philsophical/religous cirlces for hundreds of years.

What was unique about the Jesus movement was that the teachings and writings were popularized over such a wide area so effectively. The only prior example I know of in history that I'd consider similar would be the teachings of Siddhartha Gautama (complete with its own version of the Golden Rule) which spread around the Asian continent at roughly 200 BC.

Yes, there were some, more than one and surely more than we know. These known ones were from times about 6 centuries before Christ.

Scriptures of Buddhism were not less revolutionary. Buddha is considered as a teacher of people and gods. Christ had never reached so far.

Avesta of Zarathustra was the first documented teaching of kindness.

Even in the Bible itself there are prophets, that were before Christ.

Being good, kinder than people around you is always hard. In times of Christ, 600 years before him or 2000 years after. Christ became the symbol of kindness for really many people, but historically he was not unique in his kindness. (Thank God.) Yanush Korczak, for example, chose to go into gas cameras of the death camp with his orphans.

As for probability, every time when some society degenerated in morals, sooner or later came somebody or somebodies, who created new moral system that originated a new society. The rarity of the Crist/St. Paul's feat is in the fact, that their feat was not forgotten. And I would say, that the fact that they could live long enough to create Christianity and even to become known personally shows that the society they lived in was far from being the most cruel one.

First of all I want to point out that Christianization did not bring any improvements in moral standards at the time. Conversely, the moral was gradually degrading whether due or not the advent of Christianity.

Ancient Rome had possibly the most extensive legislation on social care and charity at the time, which included orphanages, pensions, alimentary systems, free public schools, free food distribution and so on. This spanned all the free population but with advent of Christianity was restricted only to Christians.

Judaism at the time also had a tradition of charity and helping the poor.

Regarding many of the Jesus teachings they were either universal principles accepted by most peoples (such as helping the neighbors, which is necessary for the people's survival), borrowed from Judaism or just impractical and never implemented such as turning the other cheek (the principles of deterrent, retribution and self-defense are very ancient ones and were never abandoned to this day).

Regarding slavery, the early Christianity and the Jesus' teachings were not opposed to it. The rights of slaves were gradually improving in Roman Empire before Christian age and this pattern continued under Christianity. At the same time the status and rights of a free man, citizen were gradually degrading in favor of the rights of the nobility.

You also mentioned stoning which I suppose was a kind of capital punishment. But Christianity never opposed capital punishment, only the methods could change. Capital punishment is widely used in Christian countries up to this day.