Why would an atheist, or group of atheists, go to such lengths to try and convince believers that God doesn't exist?

What do they care? What is their point? I have a friend on Facebook who constantly posts things that point out how foolish believing is, in his opinion. (my personal favorite was when he posted, “If you believe in the Bible, then you believe the world was populated via incest. Twice.) They’ll ridicule, belittle, try to humiliate believers. Why? Why can’t they just let them be?

I once PM’d my friend asking him why he does that. He said he really didn’t know. Is it some sort of superiority complex?