Christians: If not for god, would you be a murderer, thief, cannibal, etc? Why do christians say atheists are?

I have heard several times recently (from christians) that they think 1. morality is based on religion, 2. that atheists are only atheists because we want to "sin", and that 3. atheists can't understand morality without god. Why on earth would they say that? Do christians honestly believe that atheists are all murderers, thieves and canibals (their words, not mine), or are they just promoting their religion, (and themselves), by bashing others?