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.
Why do you say it's wrong? Wantonly killing a rapist is wrong? Wantonly killing a person in direct competition to resources is wrong? How are you proving any of this? Nature doesn't show that. There are many species that survive using that exact technique.
You need to analyze your basis. It's probably something like, "I don't want to be killed, so I don't". That's fine, but why is that a good idea? Why is life good? Look around at the suffering. It could all be over tomorrow if everyone died. Honestly, atheists assume a moral view from theists. Stalin was right: kill your enemies is fine. You can't prove a moral absolute. You can only prove a feeling set that achieves an end you declare is right.
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.
Yes. Ethics is nothing more than an arbitrary complex of rules which seek an arbitrary end. The degree to which the individual approximates the system defines how ethical they are.