1. Medieval Europe was more of a Christian society than today's America, but I do not believe it was a Christ-centered society, that is a society centered around the instructions Christ left for us.
2. it may not be the only "do unto others" group, but it is the founder of that philosophy. As for a perfect society that happens to be Christian... A completely Christian society would be just that, and nothing less. It is not possible for such a thing to occur on this side of Eternity, but if everyone on Earth suddenly dropped everything and lived by the teachings and examples Christ laid down for us, we would literally have Heaven on Earth. Now, I don't know what society you've been living in, but from what I can tell, the American mentality is not "do unto others," but rather a combination of "it's not happening if it's not happening here," "What's in it for me," and "if it feels good, do it." That's what I see today coming out of this great country.
For your other point, Mefustae, I would much prefer a world where triple homicides were unheard of. It takes two people to fight. I'd rather not face evil with evil. Call me crazy, but I am a firm believer that if evil is left to run its course, much like a wildfire, it will eventually burn itself out and cease to exist. If I offer my possessions freely, I needn't worry about theft any longer. By your own logic, we should have the death penalty for even the pettiest of crimes in order to eliminate crime.
Asuko, not exactly. That would be true, but I was talking about simply why the world would be a better place if everyone were a Christian.