Why shouldn't the Germans try the agents for those crimes, given that they committed them? You yourself suggested this as a remedy for the kind of violations that agents commit.
I don't recall suggesting that the Germans should try for the CIA agents for whatever crime they believed them to be guilty, but obviously it was a possibility open to them.
Of course, once the agents left German soil all the German government could do was either seek to extradite them or kidnap them and return them to Germany to stand trial.
Clearly the US was not prepared to turn them over to the Germans, and attempting to kidnap, let alone actually seizing them, would have resulted in a diplomatic crisis of which Germany wanted no part.
There is no international court that has jurisdiction over the US government unless the US government voluntarily submits to it. Appealing to the UN would have been useless. There case against the agents depended entirely on what the US government was willing to do, or what the German government could force it to do. In terms of the latter there was nothing the Germans could do.
Now, you have to ask the German government why, if they thought justice was not being served and their citizen so terribly wronged (which apparently he was) why it didn't at least take a symbolic stand and sever ties with the US.
The answer is obvious. Seeking justice for their citizen was not worth the likely consequences of taking a stand for the right thing.
If leaders of nations that are at war do not order the assassination of one another, it’s not for any reason of law, honor, or morality; it is because they prefer that the violence of war be absorbed by their soldiers and their citizens. We don't know for certain whether or not Hitler ever sent a hit squad against Churchill or Roosevelt, or if he ever contemplated doing so, but if he did not, it certainly wasn't because he considered it a violation of the laws of war or, heaven forbid, immoral. It was because he didn't want to open a Pandora's box of leader assassination which might sweep him away as well.
The Navy Seals who attacked Bin Laden's stronghold, did just that...attacked. They are warriors, not police and the notion that they should only have killed Bin Laden in self-defense is absurd.
Apparently, nothing stopped Bin Laden from launching an attack that resulted in the destruction of two NY skyscrapers and 3,000 innocent civilians. Perhaps his being hunted for ten years and ultimately executed by Navy SEALs will be something that might stop the next madman who thinks he can defeat the US.
Hopefully this is the precedent that is set by the killing of Osama bin Laden.