The hitler example only shows that evil can be done by christians or those who profess to be christians. It also shows that chrisianity can be used as a tool for evil. It does not mean that Christianity is inherently evil.
I remember from about 30 years ago a TV interview with Albert Speer (the architect of the third reich). He was reasonable calm in discussing his involment at the upper reaches of the NAZI hierarchy until near the end. The interviewer commented on his amazement at seeing movies of Hitler playing with a dog and children about how this "monster" appeared so normal.
Speer became rather adgitated and said (to the affect) "No, no you must not think that. He was *not* a monster! He was a human being like you and I. We all have within us the capability to do those things. If we think he was somehow special and not human we forget to be on guard."
This was one of two powerful lessons I took away from the interview.
Speer was warning against the no-true human fallacy as Jar is trying to warn Christians against the "no true Chistian" one.