I get an impression that many people pretend to be nice to get some advantages in life, but at some points they drop the mask and show what jerks they are really. For example, there was this guy I talked a lot to on Bioware forums; he was very friendly, I would even call him a friend back in the days. Then, in some thread on forums we disagreed on a minor thing. I didn't give much thought to it, but suddenly he grew very brief and cold in our interaction. I asked him what was wrong, and he told me that he didn't have any more reason to talk to me since my comments in that thread demonstrated me, basically, as a fool. Of course he used a very nice way to say that, but the meaning is the meaning. And I've had thousands stories like that in my life. I've even seen people ruining other people's lives with a smile on their face.
Anyone else can relate to this? How do you deal with people like this? On one hand, they come around like nice people, so being harsh to them feels wrong somehow. On the other hand, they have a big potential of ruining your mood or, sometimes, even your life if you take the bait and don't take any precautions.
In general, I am not sure if I must be as nice to people as I am. It backfired at me so many times that I have an urge to drop all the pleasantries when I meet new people nowadays and just be mean, regardless of what they think of me. I am tired of being used by jerks pretending to be decent people.