I have watched enough court and read enough court reporting to know that speculating on the evidence based on what you've read in the papers is a good way to misunderstand the real facts and look like an ass. Still, this is a fascinating (although clearly tragic) case because of the lack of a body and the apparent lack of any medical evidence regarding death.
The wounded Taliban is described by the interpreter as alive because he was "moving" "shortly" before Semrau is alleged to have fired the two rounds. The Afghan army officer testified he was unsure if the insurgent was alive or dead before Semrau is alleged to have fired, and in any case, he appeared to have been perhaps "minutes" away from death.
One of the essential elements required to convict someone of murder is proof that he caused the death of another human being. His act does not need to be the sole contributing cause, or even the primary contributing cause. It merely needs to be a "significant" contributing cause of the death. On the facts as I understand them, if the insurgent was alive, Semrau's shots were certainly a significant contributing cause of the death. The shots may have only hastened death by a few minutes (or even seconds), but they were nonetheless a significant cause of death.
Still, one cannot murder someone who is already dead. A person can have the mental state necessary to commit murder (intent to cause death, or intent to cause bodily harm one knows is likely to cause death while reckless about whether death will ensue), but if they do not commit the guilty act (a voluntary unlawful act that is a significant contributing cause to the death of another person), they are not guilty of murder. The Crown must prove the guilty act beyond a reasonable doubt. So regardless of whether the insurgent was "probably" or "likely" alive at the time the shots were fired, unless the jury is convinced beyond a reasonable doubt that the insurgent actually was alive, Semrau is not guilty of murder. Given the severe injuries described in the newspapers and at least one witness's uncertainty, I am not sure the Crown has proven its case on this point. On the other hand, if the evidence of the interpreter about movement shortly before the shots is believed, perhaps the Crown has proven its case. The exact evidence, and the credibility of the interpreter, becomes key.
To be guilty of attempted murder, Semrau must have committed an act that went beyond mere preparation to commit the offence (the guilty act), and Semrau must have intended to kill the dying man (the guilty state of mind). The dying man must also NOT have actually died from anything Semrau did. This would be the case if he died of his injures before Semrau fired the shots, with Semrau mistakenly believing he was still alive.
Having said all this, I have no idea what the witnesses actually said, and I'm clearly armchair lawyering, so take this for what it is worth. But if the jury returns a verdict of not guilty on the charge of murder but guilty on attempted murder, I think I can see a path to their conclusion.
Attempted murder carries no mandatory minimum penalty. In theory, Semrau could receive a suspended sentence (with a fine or a period of probation). Attempted murder carries a maximum penalty of life in prison. He would most likely be eligible for parole after seven years if he received a life sentence. Second degree murder carries an automatic penalty of life in prison, with parole eligibility set by the sentencing judge at between ten and twenty-five years.
As for why you don't call evidence... well, here, Semrau would no doubt be cross-examined on why he apparently believed the insurgent was alive (otherwise why shoot him?). If Semrau's evidence was that he saw the insurgent still moving, this would not be very helpful to his case on the murder vs. attempted murder issue, would it? (Recall he is both a Christian and a soldier. His religious and moral obligations categorically rule out lying under oath, and taking the stand if you're going to admit you're guilty is not a great strategy. Insisting that the Crown prove its case without your evidence, on the other hand, is a perfectly legitimate and moral course of action.) As for other witnesses, probably everyone with information relevant to the case had already been called by the Crown.
