The time machine worked, but it only worked in one direction. You could only go to the past. And there was no coming back. So if John really wanted to use it, he had better do something important. He tried to think of wrongs that he could right, atrocities he could prevent, evil he could avert.
He was a child of his generation. The greatest evil? Hitler. The worst atrocity? The Holocaust. It was obvious. If he could use the time machine for only one thing, he would use it to kill Hitler and prevent the evils he committed from happening. What that would mean for history -- how it would change the resulting past and create a different time line -- was an interesting question, but one John wasn't able to generate much concern over. Stopping Hitler took precedence.
It took a full week for the machine to power up sufficiently for a jump of that many years. He took the time getting the right clothes and haircut and learning as much of the details of German culture of the time. Fortunately John could already speak German, otherwise he would have had to wait months or years before he used the time machine. Of course, he'd arrive at the same point anyway, but he was anxious to get there and finish it.
Everything went according to plan, but it took quite a bit of time getting to the point where he would have access to Hitler, longer than he wanted to spend. He used his knowledge of future events to become rich and donate much of his money to the Nazi party, slowly gaining access to its more important and higher-up leaders. Eventually he was able to maneuver himself into a private appointment with the Führer himself.
John had never killed before, but this was Hitler, after all. He walked into the room, and as the door closed behind him, the man he had seen in a hundred news reels walked towards him, with his hand out. John took his hand to shake it, and with his other hand took the knife he had up his sleeve (actually he had one up both sleeves, just in case) and stabbed him in the upper abdomen, pushing it up hard, as high under his rib cage as he could.
Hitler fell to the floor and looked up at John with a look of pure gratitude. Then, as his eyes rolled back in his head, he collapsed and exhaled a long breath. He didn't breathe again. John stared at his body for a full minute. He hoped he could escape, but he wasn't really expecting to. He was willing to sacrifice himself for this greater good. He turned toward the door.
Two men were standing right behind him. He hadn't noticed them in the room when he came in, and he hadn't heard any doors opening. But they had probably been hiding somewhere to protect their Führer. Caught already.
"Sir, you are guilty of a horrendous crime. You will have to pay the penalty." Something was wrong with what he said, and it took a few seconds for John to realize what it was: the man hadn't spoken German. He had spoken John's own language.
"Wh...what? What do you mean? Who are you?"
"We are time police. Our job is to protect history by ensuring that people do not travel back in time to alter the past. The repercussions of such an act would tear the universe apart."
"But..." John began. Then, he shook his head as if to clear it and said more forcefully, "I'm afraid you're too late. I've already killed him. This monster will never live to kill six million Jews. It's over."
"Sir, we couldn't arrest you before you committed a crime. We had to let you actually do it in order for you to be found guilty."
"Found guilty by whom?"
"Us. We have the authority to arrest and punish, to confine and convict."
"Fine. I'm willing to suffer whatever penalty you can think up. It's a small price to pay to rid the world of this devil."
They looked at him in pity, and said, "Sir, there is a mirror on the wall to your left. Would you please look into it?"
He did. He opened his mouth, and the reflection did the same. His eyes widened and so did the reflection's. He raised one arm and waved it around, the image in the mirror copying his act as he did it.
The image of Hitler.
He looked to the ground where Hitler's body had fallen. It wasn't there. He and the two time police were the only people in the room. He looked back into the mirror and saw his mouth, Hitler's mouth, working as he tried to say something, anything. Finally, he managed a single word.
"No."
"I'm afraid you don't have a choice sir. You have been found guilty of trying to alter the past. That is a crime against humanity. In order to protect the time stream, you will take the place of the man you sought to kill."
"I won't do it. Even if I look like him, I won't give his orders. I won't commit the crimes he did. I won't..."
"You have no choice. You are locked in to his destiny. You will find yourself giving the orders. No matter how desperately you wish to stop Hitler's atrocities, you will commit them yourself. You will not be able to resist."
"No, please. I'll do anything. Please, don't make me commit such horrendous acts. Please, please, please!"
"You have no choice. It is your own fault for trying to change history, effectively trying to uncreate the world that gave you birth." They both reached down to their belts and pushed some buttons John hadn't noticed. They began to shimmer, as if they were disappearing. "We leave you now to your doom."
John cried out, "Wait, please! Isn't there anything I can do?"
The time police slowly faded from sight, but the voice continued a few moments longer. "Yes there is something you can do. You can hope and pray that someone else will travel back in time to kill Hitler as you did. Then they will kill you and take your place. That is what your predecessor did. And the two hundred thirty-seven before him."