I'll pop up from long-term lurking to answer this one. As you intimate, the answer is, of course, 'never, under any circumstances'. Reality is, of course, that soldiers are - like everyone else - subject to the failings of the human condition, and are perfectly capable of giving in to anger, fear, malice or a host of other 'dark side' traits. But the ROE, and LOAC, are entirely unambiguous in this area, and there's no excuse for anyone not knowing that. CSgt Blackman was convicted on his own words: he knew at the time that he was committing a crime. He didn't subsequently have his conviction quashed: it was reduced to manslaughter in recognition that his actions were not those of someone in full control of their faculties: i.e.diminished responsibility. A great deal else went wrong to get him to that point: he was evidently unwell, and was (in my opinion) let down by those in command who failed to recognise it. Again, not easy, but some pretty well-reported criticism of that unit at the time.
And that is what causes me such disquiet concerning the SASR. Commanders from 'Lt to Lt Gen' might well have known nothing about it - but they damn well should have done, and if the 'warrior culture' (a fetish I despise) had become out of control it was because officers allowed it to, through sins of omission or commission. I don't for a minute imagine it couldn't happen here, either - to units of all sorts. That's rather the point: it's on us to lead and keep leading, and to keep ourselves honest. Either way, very painful times for our Aussie brothers. There but for the grace of God etc.
Edit to add: there is a world of difference between the true heat of battle, in which there are many documented cases of the surrender coming 'just too late' as troops with their blood up sweep in with bayonets, and the cold-blooded acts that are the subject of these allegations - no weapons, bound and helpless.
Well said Sir, especially the bold.