To those who ask "how can you be good without god?"?

Often during a religious discussion, I am asked how I can be good without god. As though without religion I may go on a brain eating spree any day now. My question is to those who have thought that: Are you really only good because you believe in god? If "yes", does that scare you? Because it scares me. Am I walking around with 85-90% of the population on the brink of homicidal mania?