Hitler was probably the greatest liar in human history, so his proclaiming to be doing the "Lord's work" in murdering Jews doesn't "prove" IMO that he was a Christian or even that he considered himself to be one. The only "god" Hitler seemed to believe in was himself.
It's my opinion that it really doesn't matter whether or not Hitler was a Christian. What mattered is whether the German people believed he was, and if they did believe that he was did he use it to manipulate them?
I think the answer to both of those questions is yes. That's why religion is frightening when used by the government, and it's why I strongly dislike Bush and those like him constantly talking about doing God's work, etc.