Okay, so I've seen atheists say before that they wouldn't mind having a completely atheistic world, because they don't need religion for morals. Wouldn't that technically only work if the people in that world were good people by nature? Like what if the people were really bad, and the society started to get more and more corrupt? Then what would atheists do to get the society back to functioning?
Yosef, you are confusing "Atheism" with "Materialism," and "Immorality."
Just because a person is an atheist -
meaning they are indifferent to or denies the existence of a god or gods - does not always mean that the atheist is a materialist or that the atheist is immoral.
The disbelief in god really has nothing to do with the ethical conduct and behavior of a person.
Theism and religion does not always mean a theistic person will be "moralistic" and ethical. There are many fine examples of theists and god fearing people who commit heinous crimes and perpetrate immoral behavior and use their religion or gods - or misinterpretation of religion/god - as justifications for their actions. For example consider for instance the trouble the Vaican/Church is in in regards to child molestation and cover ups.
There are a lot of spiritually inclined atheists who are moralistic and socially conservative by natural disposition.
How we carry ourselves, how we act, and what we do is not really borne from one's belief or disbelief in a god/gods. Such things as behavior, action, and our ethics and morals ultimately arise from our inner Ethos and inner nature.
Ethical behavior and morals are often independent of religion.
For example, both the Republican and Democratic Platforms have a certain amount of ethical and moralistic ideologies in them - the Republican Party more then the Dems.
If you join the military, there will be specific sets of codes of behavior (morals) and ethical concepts to follow such as Honor, Loyalty, Duty, etc...
Institutions sch as Freemasonry and the Boy Scouts are religions, but each teaches its members a morals and ethical conduct.
Althoe I do value the benefits of religions, a world without religion or theism does not spell chaos and anarchy.
Like you say, people are good by default. Like a dog is good by nature. But if you abuse the dog, and back him up into a corner, cage him up, the dog's emotions and psychology will be affected causally, and the dog will change and react very differently.
To answer your question you would seriously have to contemplate (samadhi... vipassana) on the origins/CAUSE of unwholesome (or bad/corrupt) behavior.
In Buddhism, we understand that there is a Cause to everything (karma). Our thought influences our emotions... and those emotions governs our actions. Such that what acts (karma) we commit in the world of causal phenomena has its cause/origins.
Thus bad actions just do not come into being because of a lack in the belief in gods. Blaming our karma, bad actions, and the fruits of such actions is only part of the problem, i.e.: blaming the world's "evils" on a "Satan."
For example as to where "bad" actions come. We can say that killing and treason and crime are "bad." Then when we look at something like the American Revolution where colonialists were killing British soldiers, committing crimes and reason against the crown of England; we can see that such acts are borne from the colonialists Mind... the thoughts and emotions... thoughts of liberty, and the desire to be free from tyranny. Therefore their acts are not borne from atheism or a lack in a belief in gods.
This is only one example. I'm sure you can get yourself to think of more if you seriously wish to understand the actual cause of immorality and unethical actions.