This is The Pacifist by James P. Hogan. It can be read here.
A few relevant passages:
The larger of the two looked Kunz up and down. He had heavy cheeks and a thick black mustache, and a fleshy sausage-neck overflowing from his collar. "Don't tell me," he said amiably in German, "You've come back from a future age to assassinate the Führer."
Kunz gulped disbelievingly. "How . . . how do you know?" he stammered.
"Oh, they've been showing up in dozens all night. You'd better come with us. The line starts a block farther along the street."
which leads to
"What's happening?" Kunz asked, finding his voice at last.
"Why, the Führer is coming here to talk to you. He's heard all the terrible things you people are saying about him, and he's very upset."
which leads to
As Hitler spoke on, the atmosphere around the room changed. One by one, those of the audience who were on their feet sat down. Many of them exchanged puzzled looks. Something was very wrong. Was this the fiend who had gone down as one of the arch-villains of history?
"I must confess to being somewhat bemused," Kunz heard the English voice whisper behind him. "He seems to be quite a decent sort of chap, really."
"Yeah," the Brooklyn voice breathed in reply. "Dis ain't de way I hoid it. Dat guy's okay."
which leads to
Kunz looked furtively around. Everyone's attention was on what Hitler was saying. And then Kunz's hand was reaching out stealthily toward the butt of the revolver. He was aware of it in a strangely detached kind of way, as if his arm had initiated the motion of its own accord, with the rest of him a spectator. And before he had fully realized it he was on his feet and leveling the gun between both hands.
which leads to
"Oh dear, oh dear . . ."
". . . three bullets, dead center. Didn't have a chance . . ."
A tired but triumphant smile crept onto Kunz's face. It was done! It didn't matter what happened to him now.
which leads to what's hidden behind the spoiler.
