The situations are not the same. The Taliban Semrau shot was a badly wounded man, likely in severe pain and dying of his injuries.
The Wehrmacht Officer in question was a surrendered soldier, a POW.
And they were totally different eras, different wars, with different cultural rules.
On the whole, I think what Semrau did was, as I said, both honorable and illegal. That didn't give the Canadian military a lot of wiggle room, no matter what the fundamental ethics of his actions may have been. Notably, he was convicted of the equivalent of "Conduct Unbecoming" - not of murder. There was no corpse to be found, the physical evidence was long gone by the time the investigation got underway, and there were no direct witnesses to his actions - or at least none that would come forward. He was demoted and dismissed from the military, and not imprisoned.
In the other case being discussed, I doubt very much that any body was ever found, at least not that could be directly associated with the incident. Legally, no body = no murder, in almost every case. In the chaos of the campaigns on the Western Front, and the mass interments of German soldiers and officers... There was a war to be won. Nobody was asking deep questions that could be avoided. If an arrogant German officer went missing... I just don't think anyone would look too hard for answers about it.
That was the reality of what was going on at the time.
And again, I don't really want to get into judging either man particularly. I wasn't there, wasn't faced with their pressures, concerns, fears, and choices. I'm quite happy to have the luxury of never having had to face those choices.