Ever religious person on the entire planet lives by the tenets of their faith that they want to.
Hitler was a Christian. He said he was Christian. If, as you say, "Jesus decides" who is or isn't Christian, then *you* can't say he wasn't. You could make a reasonable argument that he wasn't sincere in his faith.
The fact is, he used Christian dogma to sway the very Christian population of Germany to his cause.