It might be argued that claiming the existence of an all-mighty God watching over us and punishing the evil-doers is helpful in trying to contain the simple minds who don't understand the simple beauty of Kant’s Categorical Imperative or the even easier Golden Rule and therefor need another moral reason not to kill their neighbours. While the claim of the existence of such a being surely helps keeping your neighbour peaceful, the need of keeping him peaceful with moral reasons might still be doubted since there exist laws in nearly every state of the world prohibiting the killing of your neighbour, and even in those countries where such laws don't exist or aren't enforced (e. g. Somalia) it is still feasible not to kill your neighbour because usually, this brings along more problems than it solves (in some countries, you even have to marry his wife afterwards!).
While the arguments given in paragraph one might not satisfy you, the extreme waste of human resources that has been caused by religion for the last millenia unarguably exists. Think of the great things Egypt pharaos could have done if they didn't had to have huge graves. Think of the many problems different religions have caused in the Middle East. Think of the many christians that died during the Persecution of Christians or while trying to get back their dry, empty little place of holy land. Think of the Thirty Years War. Think of the reason the current peoples of Pakistan and India are still willing to fight each other. Obviously, religion does not only have good results :)
You should note that the reasons given only apply to the claim of the existence of a god. They don't tackle the problem wether or not such an entity exist. Therefore, here is my “proof” of the non-existance of such an entity independently of the effect of the claim of its existence.
There is no God.
Proof: Assume that there is a God. This entity then either interferes with our world or it doesn't. If it does interfere with our world,
we can – though maybe inable to explain it – collect data and describe the results, i. e. start understanding it. This is a contradiction since all main
religions state that their God(s) are beyond human understanding. Therefor, the assumed God does not interfere with us. But if it doesn't interfere with us,
how do we know it exists? We don't, but we also don't have to know, because assuming its existance or denying it has no consequences. Since the void is always
simpler than an entity different from it, according to Occam’s razor, denying the existence of a God is the right way to go. QED.