The behaviour of religious believers or of atheists is no guide to anything. People are people: some are bad and some are good, whether they are religious or not. Religion helps many people by providing a consistent framework for their interactions with the world. Some other believers have found in their religion a spurious justification for the worst behaviour imaginable.
Agnosticism is not simply the state of being undecided about the existence of God, nor is it the state of reserving judgement in the absence of evidence. Technicaly, "agnosticism" is the belief that it is fundamentally impossible to prove or disprove the existence of God. This is itself a respectable religious position: that as you cannot prove that God exists, there is virtue in believing through faith alone.
The idea of god(s) includes covers a spectrum of beliefs. At one end of the spectrum is the pagan god of a waterfall, pond or cave: one god among many. Such a god has powers that may be very limited and specialised. At the other end of the spectrum is the sophisticated modern concept of the monotheist's God who is omnipresent, omniscient, and omnipotent.
There are also some beliefs that sit to one side of this spectrum, one example being pantheism: the belief that all of reality is identical with divinity. Instead of God creating and ruling the universe, God actually is the universe.
An omnipotent God would have the power to do absolutely everything, all at once, everywhere.
Somewhere part way along the spectrum is the god, such as Zeus, who has what you might call "limited omnipotence". He can do any single thing he chooses, but not everything. He can cause an earthquake or a tidal wave, or change himself into a bull — but while he's busy doing that, he can't be doing something else as well. It's like having a million Pounds to spend: you can buy any car you want, but not every car you want.
When considering an atheist's position, you first need to ask which of the above sort of god(s) they reject, and why.
I imagine most of us here (but perhaps not all) would reject the idea of a "god of thunder" who causes the thunder with his big hammer. Most of us would reject the idea of the god of the sun, riding across the sky in his fiery chariot.
We reject this type of god because we have better, evidence-based, theories for why there is thunder, and why the sun crosses the sky as it does.
An important aspect of these modern theories is that they enable us to make reasonably accurate predictions. We have a scientific model, we can feed in data and produce a forecast. We can observe what really happens and then refine the theory. Belief in a thunder god does not enable us to predict when or where there will be thunder. Meteorology allows us to make pretty good weather forecasts. Therefore, the belief in thunder gods has been replaced by a demonstrably better explanation.
Now, let's look at this idea of the best theory being the one that helps us to make predictions.
The sophisticated modern view of the monotheistic God is that He is omnipresent, omniscient, and omnipotent. He is everywhere, knows everything, and can do everything. He is also considered to be ineffable: we cannot possibly know His thoughts or understand his motives. The Lord works in mysterious ways: he causes the devastating tsunami but saves one child who is swept away — and we can never know or understand why.
If you have such a concept of God, then absolutely every possible set of circumstances, every possible event, can equally be explained by the existence of God, and by God's will. The child dies, God called him home; the child lives, God saved him.
If every possible set of circumstances can equally be explained by the existence of God, and by God's will, then our belief in God does not enable us to make any predictions of any kind, about anything. The existence of this type of God is not a testable hypothesis because there is no conceivable set of circumstances that could only occur if God did not exist.
If you follow this line of argument, then it is not a case of whether or not God exists, it is a case of "a stupid question". You might as well ask what Otzi the Iceman's real name was; whilst it is likely that he had a name, because personal names have been used in all or most societies, there is absolutely no way of either verifying or falsifying one name or another.
And that is why I consider myself to be an atheist: because "Does God exist?" is not a useful question.
