Not sure if it was ever confirmed, but I always heard multiple countries could have easily killed Hitler, it's just that he was a very very shitty military commander, and the nazis would have been better off without him.

He would overrule his incredibly intelligent generals and make stupid emotional decisions, without him the war would have stretched on much longer.

The war didn't end cuz Hitler shot himself, he shot himself because the war ended. People get that mixed up.