Those who believe there's no basis for ethics without a god, or at least a religion, can't see there's a downside to violating others absent a supernatural referee.
I understand why they might feel that way. It's the same sort of thinking behind acceptance of government courts and police. Why not be a thug if no one is looking over your shoulder and holding you accountable?
If there is no one keeping cosmic score, why not just go through life using other people as you see fit? A completely "me first" attitude in everything?
Because while it often works as a short-term strategy, it falls apart in the long-term. Living this way is unsustainable. In fact, governing is the only way to survive such a strategy beyond the short-term. Live completely free of regard for others, on your own, and you're soon dead. Not only will people defend themselves from you, not only will people soon refuse to deal with you in any way, but other people will adopt your strategy and use it against you. Society beyond the rudimentary would be impossible. This life plan is self-defeating.
Ethics is a real thing and doesn't require a god or State to enforce it. It arises spontaneously out of living among other humans who have the same needs, desires, and feelings as you do. Your behavior has consequences.
Nothing's perfect-- some people will always seem to get away with crimes against others-- neither god nor government will change that. In fact, government makes it more likely that they will get away with it longer than otherwise. Government protects the bad guys from the consequences of their behavior much more than it protects the innocent. It always has and always will.
This illustrates why government-- the State-- is the opposite of society, not a synonym.