While many things helped me realize there is no God, the clincher is the Indonesian tsunami. All those children not old enough to choose to move away from such an environment, swept away by something any omnibenevolent superpower could have/would have prevented.
In "The Brothers Karamavoz" a main character basically says that even if God fully explained why all the suffering had to take place, and the explanation was totally satisfactory, the price is still too high for His reasons. The main character said "I return my ticket" meaning he would rather not be any part of any purpose God has for Man.