The Allies thought of assassinating Hitler but they went off the idea because they feared he would be replaced by someone actually competent at the job.
That's true.
If Hitler hadn't meddled with the War Plans then the Allies might not have won the war.
More like he had all the military power in his hands, and none of the various generals could do anything without his approval. Remarkable feat obtained by laying down a hierarchical chain of command that was a waking nightmare of overlapping competences and responsibilities that made so that no one but him could ever have the last word on ANYTHING. All of this because of his belief that struggle would make for better fighters.
In short; he promoted infighting between his own ranks! The worst enemy and double agent of Adolf Hitler was Hitler Adolf.

How the hell was her going to win against a sanely structured army is beyond anyone's guess, since EVERYTHING he ever did went in the face of the most basics rules of strategy. Rules acknowledged more than two millenniums ago already, in that treatise of rule of warring called "
The Art of War" written in China and commonly attributed to Sun Tsu.
The only things he got right, to be fair, was the developing of a war always in motion instead of WWI-style trench warfare (
the infamous blitzkrieg), and the aerial support of the infantry by diving bombers (
only way at the time to have some sort of precision on where the payload would land), in all the rest he was an incompetent moron... thank God for the small blessings.
A question that is hardly never asked, but what would the World/European population be today without the intervention of Hitler, or Stalin, or Saddam, in fact any dictator is history. Unsavoury I know, but war is a great way of thinning out the virus that man is...
The second world war would have happened anyway. It's widely acknowledged that the way he first world war ended, especially between the French and the Germans, was just a scurrilous invitation to a re-igniting of the belligerences in less than two generations... and it happened.
Had an Adolf Hitler not existed... the war would have gone in wildly different ways impossible to fathom here and now, but would have happened all the same.
There is no such a thing as overkill. Only unworthy targets.