Massmurder will occur
I'm not particularly interested in advocating atheism but sometimes, like after reading the thread that GilDodgen at uncommondescent started (22nd Nov, with it's following comments), I feel the need to. The discussion starts out fine enough, but quickly degenerates into a "if there is (or if people believe there is) no absolute morality then people will kill eachother indiscriminately" kind of argument (as argued by for example DaveScot). I'm not going to write an exhaustive rebuttal to this, but instead just supply some points for you to consider.
1. Isn't this a rather grim view of humanity? Whether you are christian, muslim, buddhist, atheist or jedi, think about it. Are we to believe DaveScot and friends when they say that the only reason we don't kill indiscriminately is because we know someone is watching our every move and has the capacity to severely punish us? The reason you don't kill that annoying workmate, person at school or guy on the cellphone is hardly because you think that you will burn in hell for it. You don't do it, for the simple reason that you KNOW that it is wrong (or more likely, perhaps, it never even occurred to you). Whether or not that knowledge is based on something supernatural or is a result of evolutionary forces (and/or lots of other possbilities) hardly matters. DaveScot's friends would have us believe that only the threat of severe punishment deters murder. That's the reason sociopaths don't do more bad things than they do. The rest of us just don't do them.
2. If you compare theory (atheism->evil) to reality, an interesting picture emerges. In the US, around 75% of the total population is christian. Around 75% of the prison population is also christian. No surprises there. In the US, around 3-8% (depending on source) of the population describe themselves as atheists. Around 0.2% of the prison population also describe themselves as atheists. It would actually seem like atheists are LESS likely to indulge in criminal behaviour. Why would that be? You can always argue that maybe atheists are less likely to go to prison because they are less likely to get convicted (you can even insert your favorite conspiracy theory to account for this, if you wish). You can argue that atheists commit more serious crimes (but if you do, you'd better back that up with some data). You could argue all sorts of things - maybe even the thought that atheists simply are NOT evil and that atheism does not lead to bad behaviour. Do you reckon we would ever find DaveScot et al doing that?