[Atheists] do not believe in the existence of a soul.
Although most Buddhists do not believe in any gods, and can therefore be described as atheists, they most certainly do believe in the concept of a soul — indeed, a soul that can continue to exist after death.
You do not believe in any source for ethical conduct except sources within human beings
There have been several good ethical theories put forward that go further than “look within yourself and you'll know what's right”. Arguably, the two that seem to gel best with current observed human ethics are Kantian ethics and Utilitarianism.
Kantian ethics look at the means rather than the ends.
- We ask, should one do X?
- We imagine a world where everyone does X.
- Is this world flawed?
- If so, one should never do X.
- If not, one may do X.
That is, if we want to know whether it is morally acceptable to lie, we imagine a world where everyone lied all the time. Nobody would be able to get anything done in such a world. (Imagine a trip to the shops: “that'll be ÃÂÃÂ£10.49”, “really?”, “no… actually it's ÃÂÃÂ£8.49”, “ok, here's ÃÂÃÂ£10 — keep the change”, handing the shopkeeper a ÃÂÃÂ£5 note.) And so Kantian ethics says that lying is wrong. It doesn't consider the fact that some lies may make people happy. (“Love the haircut!”)
Utilitarianism is the philosophy that one should do what increases the world's total level of happiness. So while, say, all murders might be deemed unethical under Kant, utilitarianism says that murder is OK, provided it increases the world's total level of happiness, so it may well be that murdering an internationally reviled figure (e.g. Hitler) would be deemed ethically allowable.
I think that most people tend to sit somewhere between those two extremes. There are certain actions where they take a more Kantian viewpoint (e.g. murder is always wrong), and others where they take a more utilitarian view (e.g. it is OK to lie if it makes people happy and doesn't harm anyone). One does “look within” to find ones own personal balance between these two ethical codes.
But that's not to say that tomorrow, or the next day, or some other time in the future, some bright philosopher will find a code of ethics that unifies Kantian and Utilitarian ethics in a non-personal and non-arbitrary way, and publish a code of ethics that everyone would be prepared to subscribe to.