What exactly is sin?

I must say.. I'm getting rather tired of being called a sinner.  Just the other day a theist (Christian, of course) that I didn't know from a bar of soap stopped me on the sidewalk on my way to get lunch.  He intended to give me a small booklet containing a whole bunch of bible verses.  I've seen hundreds of them before in my life and I respectfully declined.  When he asked me why I declined the book, I told him in the friendliest and most respectful manner humanly possible that I didn't want the booklet, because I never bought into any religions - I'm an atheist.  This brought on a very disgusted expression on this chap's face, then he called me a sinner.  My first instinct was to give him a fat klap, but I resisted and carried on my way.

On my way back to work I started remembering other times religious people accused me of being a so-called sinner and this has happened MANY times before.  In forums, Facebook pages, bars and social gatherings, by just giving that one tiny piece of information (I'm an atheist), complete strangers have assumed and accused me of being a degenerate lowlife dishonest indecent trashy criminal who does not deserve to breathe.

This makes me wonder... I give to the homeless, I don't steal, I pay my taxes, I respect those who have shown they are worthy of respect, I have never cheated on any woman before and I don't intend on ever cheating on my wife, I take good care to help anyone who needs it and I try my best to not be a dick towards anyone.  I consider myself a good, kind-hearted person.  My understanding of sinning is doing bad things, but clearly according to christians I am a sinner and I'm not worth spit.

Christian News24 readers, please clear up my confusion.  Educate me.  What precisely is sin?  What makes people who do not believe in the deity of your particular choice such evil, evil people?