Outlined rather well by http://www.godlessgeeks.com/LINKS/GodProof.htm, it goes something like this:
"(1) Person X, a well-known atheist, was morally inferior to the rest of us.
(2) Therefore, God exists."
It's just poetic, isn't it?
So... Assuming Hitler was an Atheist* - does this shed any light on the existence of God? Well, no.
There are a few separate points to address here, and I'll take them in turn.
If a bad person believes something, it is wrong because they are bad.
Imagine that no-one knew what water was made of, chemically. Then imagine an utter arsehole suggesting that hydrogen and oxygen played a part, and that he could prove it using reliable, empirical data.
Would the data - the real-world physical indicators of credibility - lose weight because the person presenting them was a bit of a dick? No. Those two things are as unrelated as a person's telephone number and hair colour - i.e one thing simply doesn't effect the other.
If you're unconvinced, consider the flip-side. If a really nice, charity donating, community supporting, Bible-bashing Christian attributed their honest nature to a sincere worship of what they imagine their God to be like, would it shed any light on the question of the existence of that God? Again, no - the same way that the entirely subjective cuteness of kittens doesn't either.
So, moving on... (Read more of this post)