Already Killed Him
"I've invented a time machine and I'm going to teach you it's principles", said the old man.
"Wait. If this is true you should have went back in time and killed Hitler", I replied.
"Already did", the old man said, "After the device started working I went back to the 1939 and shoot him".
"But we know Hitler lived till 1945…"
"That wasn't Hitler. That was me. Turns out without Hitler America starts a much more cruel war with Europe several years later. I had to assume the guise of the fuhrer and redo everything he did."
"Then you should have jumped to the onset of some epidemic and brought them cure", I suggested.
"Did that several times, nothing good came out of it. By the present time the humanity was wiped out by wars and overpopulation."
"But we're still alive!"
"That's because I went back and delivered a new, more deadly strain of the virus every time."
I paused for a bit, thinking.
"Then… you could have prevented the collapse of the Soviet Union"
"Tried that, I was the one who brought it down in the end."
"Death of Pushkin!"
"I was the one who shoot him in the end."
"Twin towers."
"I trained the pilots!"
"Middle ages inquisition…"
"Guess who started it all."
"So no matter what you did, it was all for the worse?", I asked, "If I were you I'd think twice before teaching me the secret of time travel, for the one evildoer I should probably go back in time and kill turns out to be YOU."
"Ha, ha", the old man replied, "Why do ya think we're so similar in appearance? You have already done that."
Before the main character sits the main character himself, only aged.
Long time ago someone invented the time machine. He went back in time and killed Hitler. But without Hitler history went for worse, and the inventor had to assume Hitler's position for five years.
In this way, it turned out that Hitler never waged any war at all! The inventor did. There's a stable time loop, in which, in trying to prevent his own atrocities, the inventor goes back in time, kills the suspect (Hitler) and commits atrocities under his name by himself.
Hearing of this, main character of the story goes back in time and kills the inventor of the time machine.
But turns out, there's a stable time loop: the inventor's actions were actually performed by the main character himself. He has to continue doing all that he thought the inventor did (killing Hitler and assuming his position), so that the history doesn't turn even worse.
Q: If the aged main character tells the young main character about this beforehand, why would the young main character still go back in time and kill the real inventor?
A: Of course because if he doesn't, the history turns for even more worse 