I think this is a key element in the story. The seemingly subtle change from an oath to support the state to an oath of loyalty to Hitler effectively defanged the officer corps. Remember, the German officer corps had a long tradition of loyalty to the state, regardless of the ruler. As Clint says, the concept of oath-taking was serious business to the German officers. It went against everything in their training and tradition to violate their oath. By changing the allegiance to Hitler, rather than to Germany, it effectively meant that their choice came down to violating their oath (and their conscience), or continuing to honor their committment. Obviously, many chose to honor their committment. We benefit from hindsight by claiming that it would have been more honorable to violate their oath, but my guess is that, at the time this was a terrible choice to make. Failure to maintain that oath, and an unsuccessful assassination attempt meant certain death, probably in a horrible manner. Who knows what really was in their personal consciences. Survivors of WW2 might suggest they weren't really loyal, but feared the consequences of action. I don't know that I can second guess them 65 years later.
Hitler's distrust of all others, in any position of authority in their respective branches was perhaps behind this isolation as much as anything. Plus, with the top officers not knowing who had what where, they would always be subservient to him for their plans and such. The different branches (sea, air, land) also had no cross communications. The Luftwaffe wouldn't allow aircraft to aid the Kriegsmarine. Goering always insisted that if it flew, it was his domain. Even the communications between the Luftwaffe and the Heer was at times confined to the local tactical needs, not the strategic needs of the land forces. A real batch of doo-doo when it came to co-ordinated command structures. The rivalry between the services is nothing new, the Germans and the Japanese made our (American) conflicts between the services seem a minor tiff between family members in comparison.
In fairness, there were many generals who sided with Hitler when things were going well, then when things began to go soure...... Many generals including Manstein, tried to distance themselves as far away from Hitler as possible after his death in order to conveniently mask their own blunders. For example many of the Generals (Keitel included) thought that Russia would capitulate within 6 weeks.... Hitler gave 3 months. After the war however, none ever admited to these calculations and blamed Hitler for poor planning.
Exactly! Hitler surrounded himself with his own elite version of the SS and used the SS as his personal army... Hitler never trusted the regular army. Great question... Eichmann would have followed Hitler to the end of the world and just about did. In the Valkerie movie they really hammer home how much of a hold the Oath of Loyalty to Hitler had on those involved... With all the death going on... who would dare speak up?
Another book that helps explain this is Panzer Battles by maj Gen F.W. Mellenthin. He describes numerous times when manstien flew to hitlers headquaters and threatened to resign if hitler did not change his policies. there is no doubt that hitler cost countless thousands of lives needlessly. But simply put yes there were many generals that wanted Hitler gone. But Hitler had a habit of disposing of generals that didn't see things his way. So many generals felt that they were saving lives by not saying anything and keeping command of there units. There was a tendency for generals of lesser skill replacing generals.
"At all times General von Hünersdorff was both a fanatical enemy and despiser of national socialism. He made no secret of his feelings. In his command post always on display was a picture, showing a flock of sheep following a ram with the caption : FHQ ( Führerhauptquartier)." From The memoirs of a Panzer Lehr officer by Helmut Ritgen (14 July 1943-General-Major Walther von Hünersdorff, commander of the 6. Panzer-Division was shot in the head during Operation Zitadelle and died three days later in Kharkov.)