One issue would have been the morality of the so-called good Germans as virtually all of the figures involved in Valkyrie, especially the military officers, were active supporters and enthusiasts of a war to correct the defeat of 1918. In other words, their primary reason for wanting to kill Hitler was not that he started a war or committed grave atrocities but that he was LOSING a war they had wanted.
First, the morality of the Valkirie generals is not any different from the one of the Soviet generals that were enthusiastically fighting a war to save Stalinism or the Confederate generals that fought to make America safe for chattel slavery.
Fighting for your country is no crime as long as you respect the laws of war, and if saving your country means you have to defend a horrible regime, well it's a lousy bugger choice but sometimes it is politically unavoidable, given the circumstances. For Germany, the right political conditions to overthrow the Nazis did not realsitically materialize before 1943-44.
Moreover, it must be acknowledged that Hitler's actions to reverse the 1918 settlment were not really any immoral until he started to implement Lebenstraum plans. Any sincere democratic German patriot would have enthusiastically endorsed actions to recover Rheineland, Austria, Sudenteland, Danzig, and the Corridor. And a war to being down Stalin (who would have attacked Europe in the end, had not Barbarossa prevented him) would not have been immoral except the Nazis meant to substitute Soviet slavery and extermination with their own slavery and extermination.
It was just and moral to do whatever was possible to being down Hitler for the atrocities he had unleashed upon Europe, but realistically the conditions in Germany did not exist until 1943 (there was a fleeting window in 1938-39, but continued success of Hitler's policy closed it). Sad it may be, it is basic fact that it is very hard to pull down a leadership, however barbaric it may be against its chosen opponents, as long as it's pulling a continued string of successes at home and abroad, just for a moral point. The vast majority of people, in any country, are not that idealistic or altruistic.
Resolved: That as of the current date, August of 1944, Adolf Hitler would better serve the Anglo-American cause better alive than dead.
Indeed, if any Germans as a clique were capable of assassinating the Fuhrer the Anglo-Americans should take whatever actions they can to prevent this.
This simply makes one a willing accomplice in Hitler's crimes and atrocities, and destroys the whole Allied moral high ground, nor to mention the essence of their first war aim, eliminating the Nazi.
1) The German motive: If this is to be carried out by representatives of the General Staff and German aristocrats, and it was in OTL, the benefits to the Western allies would be nil.
Realistically, they were the only ones able to bring the Nazis down. A grassroots popular democratic rebellion in Nazi Germany in the middle of WWII was hardcore ASB. Besides, the benefits to the Western Allies (and the world) would be indeed huge: a quick end to the war, cutting down the butcher bill and the devastation, sparing many of the Holocaust's victims, liberating Europe and ending the Nazi menace with much less bloodshed, and preventing Stalin from becoming the master of half Europe (which all but destroys most of the WA's second war aim, liberating Europe from tyranny).
If they succeed, they form a provisional government and demand negotiations and NOT unconditional surrender.
There is nothing immoral nor unjust in wanting to preserve your own people from the likes of Plan Morgenthau or Stalin's atrocities, before laying down arms.
As an example, a concession regarding Austria, the Sudetenland, and the Polish Corridor.
Again, this is nothing immoral or unjust or threatening the freedom or safety of Europe in demanding that Germany has territorial unity and integrity in its ethnic-linguistic borders. Those were German lands, whose people wanted to say in Germany.
Dolchstoss/Stab in the back: Allowing this and agreeing to a negotiated surrender repeats the error of WWI, leaving a Germany subject to the deadly poison that they were defeated by traitors at home while the outcome in the field had not been resolved. Such a post-war Germany would seek revenge, first against the home-grown 'traitors' and then World War Three would be at hand.
Negotiation would be about garantees of territorial integrity, no deindustrialization, no Communist occupation, and no summary/collective punishments for the German people. The military situation in 1944 was bad enough that the vast majority of the people was not doubt thre reality of defeat, especially when America unveils the nukes later. Not to mention the fact that Gwemrany has lost TWO wars in a row. Dolchstoss was only a factor because the unjust territorial losses and reparations alienated the German people against the peace deal. If the Allies eschew territorial losses and huge reparations, let the German economy recovery, there won't be any mass revanchism. The revelation of the full import of Nazi crimes will alienate the mass of the people against the legacy of the Nazis for good, and any revanchism by association. besides, revanchism was only a problem because disarmement was not implemented.
3) A pox on them all: Far too many of the conspirators, such as Carl Goerdeler and Marshall Beck, the civilian and military heads of the conspiracy, were ready and eager to follow Hitler before there was a Hitler to follow. Why were they conspiring against him now? He was losing the war. Hardly an attitude to endorse or have much confidence in.
One does overthrow a dictatorship when the realistic conditions for doing so exist. Pure political idealism which lets one openly defy the tyrant nonetheless belongs to a rare few, which typically end up in the concentration camps and the execution camp, years before any crack in the regime does materialize. The Havels and Mandelas who manage to overthrow the tyrant from their prison are as rare as hen's teeths, in history. More often than not, tyrannies are brought down by the "impure", the moderate, sane, reform-minded members of the ruling elite who begin to have second thoughts about the wisdom and competency of the leadership, even if they cooperated since then. Gorbachev, Suarez, Deng, Jaruzelski, the 1974 Portoguese generals, the last Apartheid government. The Valkirie generals were nothing special in this regard. That a tyrant is typically brought down by second-minded associates and not "pure" revolutunaries may not fulfill messianic expectations, but generally ensures much less bloodshed and suffereing than the alternative. And more often than not, revolutions build just as bad successor tyrannies. Robespierre, Khomeini, Castro, Lenin & Stalin, Mao, etc.
From this perspective Hitler becomes a justification but the true enemy is German militarism and German expansionism. It is these and not a particular political figure who must be destroyed.
German militarism get pretty much eradicated if you enforce effective democratization and demilitarization. Savage territorial losses, partition, deindustrialization, and Communist occupation are not necessary nor beneficial. Besides, this completely ignores how Versailles caused Hitler, and WWI was born out of Edwardian imperialistic rivalries of the Great Powers, of which Germany no more and no less guilty than all the others. Gavrilo Princip was not German nor the pupopet of German militarists.
Assume the coup is given the blessing of the UK/US along with various promises which might or might not be honored. Now assume it is crushed. If this happens, is it not likely that if the Nazi Party finally reaches the point of discussing an arrangement, they will do it with Stalin?
If Hitler does survive the coup, he won't seek a separate peace with Stalin or he would have done so in 1941-43, as indeed he did not ITOL. So this leaves the WA no worse for the try. Just another opportunity that failed to materialize. Such things happen all the time in war.
6) Repeat as 5 except assume the coup is partially successful, and Germany lapses into civic breakdown or outright civil war. Also note Stalin's Red Army is much closer to Berlin...
Any German would much better prefer to surrender to the A-A than the Soviets, so it's time to throw all hesitation to the winds and exercise all possible efforts on the Western Front to advance as east as possible.
The American perspective: Assume total success for the plotters and the negotiations begin. An enormous number of Americans consider Japan to be the real enemy, and are finding the losses in Europe unexpectedly severe. Can it be confidently assumed that even FDR could hold out for unconditional surrender if an alternative is available, with the election in three months.
It could not, indeed, but again this is no problem whatsoever, it is a blessing to the world, since the kind of peace Roosevelt advocated was a huge disaster (not to mention an atrocity as immoral as the crimes it purported to avenge) only Stalin would be happy with, and the sooner FDR is reomved and someone like Truman gets his place, the better.