Dear Csaba!
What you describe sounds really tough… and as you and others have already written, it sounds like a reaction to being rejected…
I too felt like that at times, and it mostly stemmed from being rejected at school. I could not recognise it at that time, it just felt like I was surrounded by stupid people who just did not get reality. Sometimes I got so angry I had very vivid and violant daydreams. I guess I can be thankful none of that turned into actual behaviour.
What you wrote also reminded me of an autobiography I read about a guy. Don’t get this the wrong way, I am not comparing but want to show you that this may happen on an even crazier level: He was abused as a child and had a really difficult relationship to his mother. This affected his relationship to women in general. After being rejected and disappointed a lot by women, he grew very hard and cold and became an abusive pimp. He wrote about the stuff he did to women, and that it took him years in prison and therapy to find out that it was his disappointment and his lack of selflove that he projected as hatred on other people. (He now owns a fitness-studio and teaches children about self-respect, so much for turning things around ;-))

I guess what I want to say with this is: You are not alone. Your feelings don’t exist because your a bad person, but because there is something unresolved, as the others have already said.
It is truely great that you notice this yourself and ask for help!!! It sucks to be rejected, and sometimes people do it in a way that seems cruel. But this should never lead you to question your selfworth or to become cruel in return – maybe they are just the wrong people for you!
I hope you can turn this around, find people who care about you and enjoy caring about other people again!