Why would some atheists be so rude, mocking, and smug?
From a Jewish standpoint I can certainly tell you why. This describes what is more often than not the average behavior I get from Christians...and I'm a theist!
I was told by someone I thought was my friend the following after he found out I was a Jew: "Why didn't you tell me sooner? I had you over for dinner in my house, sitting at the same table with my own mother! I can't believe you let me do that!" Yep, the words of a Christian.
Mocking? Try putting yourself in my shoes when you constantly hear things like "Christ-killer!" or "too bad you're going to hell" or "you folks made up the Holocaust" or various versions of "you dirty Jew" even today in the 21st century. I don't believe I ever had one atheist or agnostic tell me things like this, but if I had a dime for every Christian that did...
And don't get me started on being smug! Imagine the audacity of a group of foreign people who have stollen your legends and many of your traditions, twisted and turned them to fit their own purposes, and then tell you that don't know your own culture or history as well as they do? That's what it is like to hear a Christian tell you "the true definition of the Messiah as found in Jesus Christ." I have had to remind some of these smug people that the word "Messiah" is a Jewish term and we Jews should know better than anyone how it is defined and of whom the word can be applied to. "But we worry for you Jews," said one lady from the Jews for Jesus movement. "We don't want to see you end up in hell because you refuse to believe in Jesus!"
I don't agree with Mr. Dawkins techniques or approaches, but I sure do understand where he is coming from. It's hard to have self control when you aren't shown courtesy or decent behavior and speech on a regular basis. I can only imagine what it is like for atheists if I have it this tough.