I think it's impossible to talk about all JW as a group or a generalization. When I was a JW, I sure knew a lot of people who thought that because they were witnesses they were better than worldly people. I also knew a lot of witnesses who thought they were better than a lot of other witnesses.
I think the Watchtower directly encourages this. Not only with their constant admonitions to be no part of the world, but also through the phrase, "bad associations spoil useful habits." How many hundreds of times do you think you heard that?
I honestly don't know if I ever felt that way. I don't remember feeling superior. If anything, I felt the opposite. And I think many, many other witnesses did not, in fact, look down on people -- they just wanted to help them in a misguided way.
I bet, however, if one conducted a cross-sectional study of any major religious group you'd find relatively similar outcomes in regards to feelings of superiority among group members.