I strongly disagree. I am atheist but have no doubt that wantonly killing people (or for that matter, animals) is morally wrong. Empirically, I don't think atheists are much more likely than theists to commit crimes.
As to committing more crimes, of course not. Why would an atheist put themselves in harms way of the masses? They know that the masses will quickly end their lives for certain crimes. For other crimes, their lives are functionally ended due to incarceration. According to the rational atheist worldview, living is good. Living as free as possible is better. Therefore only commit the crimes you know you can probably get away with.
Ultimately, an atheist can no more say, in an absolute sense, that Joker is worse than Batman.
The Joker is worse than Batman. ;) Denied.
Let me ask this - do you think an atheist can be ethical?