1804 Operation Foxley: The Assassination of Hitler | The History Network:
"Part of the reason why things had been going so badly for the German Army was perceived to be down to be Hitler himself.
Lieutenant Colonel Ronald Thornley, the Head of Section X and SOE was a staunch opponent for the assassination. He reasoned,
"As a strategist, Hitler has been of the greatest possible assistance to the British war effort. Although the military situation has been temporarily stabilised on the Western Front, Hitler is still in a position to override completely the soundest of military appreciations and thereby help the Allied cause enormously."
As can be seen from this quote, there was a considerable support for the idea that not assassinating Hitler might be a better option than doing so. Removing Hitler from the scene may well result in someone far more effective militarily taking over, perpetuating the war immeasurably...
The potential boost to morale resulting from the Martyrdom of the Fuhrer could be enough to rally the troops and civilian population to new levels of resistance. Many therefore saw it as far better to defeat Germany in the field rather than resort to other more underhand methods of winning.
It would be better in the long run for Germany to witness the decline and inevitable defeat of their leader as he blundered from one catastrophe to another than to have him cut down now. Were this to happen it could give rise to future resentment and the question as to whether Germany would've been victorious if Hitler had lived."