Mercy killings are forbidden for a number of reasons. One being it isn’t the role of untrained soldiers to make medical decisions as to which type wound is, or is not recoverable.
And for that reason, the officer was aquitted. somewhat analagous to a self-defense situation. If you use deadly force to defend yourself, be prepared to face a trial.
I presume the verdict was likely due to the presiding officers agreeing who would vote in which way so as to create a lasting impression that the defendant would have been executed, or sent away for a long time had it not been for some obscure technicality that resulted in one officer voting for his acquittal. That's my guess.