The way a lot of people act, they do seem to think they are without sin.
I think part of the problem is that for everyone, it is easy to avoid certain sins (but not others).
So people look down (and cast metaphorical stones) at people who have trouble resisting sins that they themselves can resist easily.
For some, the sin is lust. For others its pride. Personally I am with C.S. Lewis in taking the view that lust is pretty minor as sins go - but the way a lot of people act, its the worst.