I would question how educated you would have to be to know that killing prisoners is wrong.
Guards in prisons were also very poorly educated at the time and yet they knew that killing prisoners was wrong otherwise no one would have walked out alive.
The argument that they were treated differently then others who did the same thing is a good point. If it was not so long ago I would vote for rounding up the guys that got off and putting them on trial and having them shot.
In other words I do not find the Morant sentence too harsh, I find the punishment meted out to the others too light.
However it is much too late to do something like that. A pity really.
It isn't as simple as that. They certainly would have known that you couldn't normally shoot Prisoners of War.
However, Queen's Regulations did allow the senior British officer on the scene (who in this case was Lieutenant Morant) to, in certain very specific circumstances, convene a field court martial to deal with spies and similar people and there were orders circulating that said that Boer prisoners wearing British Khaki should be treated as spies.
Morant knew the special circumstances didn't apply (they were intended to deal with cases where it was impossible to hand the defendants over to proper authorities). He had actually protested similar interpretations in the past to his superiors. He also probably knew the orders about Khaki were unlawful.
Handcock and Witton probably didn't know either of those things. When Morant told them about the exceptions that allowed his actions, they believed him.