I know this because of the contradictions. If there are a thousand beliefs that all contradict each other then only .1 percent of them can be right and there are much more than that. So when it comes to finding truth or fighting for the right cause etc. people are clearly incompetent. They will follow whoever will lead them. If people are so stupid that even when they try to be good they are evil then god is responsible for that evil as it is an inevitable result of the incompetence he created us with. So god is at fault and cannot possibly be perfect (all knowing, all powerful, good etc.)
It's been said before that you should never judge a philosophy by its abuses. People are not perfect, and if we are going to argue human perfection as a requirement to prove a perfect god, then we are left with various religions which argue that their gods, much like us, are flawed with emotions.
Most religions also point to the fact that their gods try to lead them toward perfection and perfect living (Buddhism). Some religions even acknowledge their followers are wicked and living outside of the perfection of their god(Judeo-Christian).
Some things are objectively perfect such as scoring 300 in bowling is called a perfect game. Other things are subjective such as the perfect spouse. In cases like that, what makes one thing perfect to one may be seen as a flaw to another. This is the type of perfection people see in their god. Example: Most Christians would probably say their god is perfect but a Hindu would likely disagree and find the traits of their god make it perfect instead. The same thing applies to branches of the main religions. A Protestant views their god as being different than how a Lutheran sees their god, even though they are basically the same god based on the same scriptures. As I have said, this means that to most people, their god is perfect in every way.
Some people who loose faith tend to blame everything on the god they did/do believe in. If they no longer believe in the god, blaming something that they don't believe exists is pretty silly. If they still believe, odds are there is some religion out there that holds a different view of the god. Instead of saying that their god did the things that are considered evil, they claim that the god was justified in doing so or they may claim that the story was not literal and just a parable or allegory. Odds are, if they look, they can find a religion like that and once again believe in a perfect god as it either did not do those things or justified doing them.
Either way, at least in the Bible, the point is made often that humans are incompetent. The reason for this is not because God made a flawed creation, but that humans decided to rebel against God. We can't be perfectly good because we are fallen and wish in our hearts to rebel against God. (Before humans fell, they weren't flawed, and although they didn't know anything, they had God with them to guide them.)
Also, humans don't know everything. Ignorance creates incompetence.
While bush was a bad president, it wasn't because he was a christian, like alot of people say correlation does not imply causation, every humans has his or her flaws and that's part of what makes us a person, and it's not solely becase of religion or anything of the sort, everyone can be bad, any relgion, any race, any family, and any specie, and nothing will change that.