There are many reasons for people being agnostic and / or atheist. I actually quite like the Gnostic way of thinking thought it was seen as a heresy by the early Christian church and believers were hunted down and killed. Nice.
But in the main by atheism is heavily reliant upon what religions tell me their god is, because it is obvious such a god cannot exist. That a god may exist in a metaphysical sense cannot be disproven or proven, but when people start to tell you what god is, that is when their foolishness becomes evident.
If you believe god is all powerful and all good, you are wrong. No all powerful, all good god could make it a principle of nature that life feed upon life, with all the bloodshed, pain, sorrow and death that such a principle necessarily embraces. And this is one of hundreds of flaws in the arguments of those who presume to describe what god is and what god thinks. If you are going to believe in a god, stop making it into a religion. That is your big mistake.