That is some twisted narrative the abc has been spinning about their own source.
If David hadn't wanted to expose the murders, he wouldn't have leaked evidence of it. What's more, he leaked evidence of their cover-up up to the highest ranks, which could be argued is he graver war-crime, since it fosters a culture of impunity.
It is true that David saw some soldiers, who served in Afghanistan the year after a lot of those murders took place, prosecuted unfairly, the way he saw it. He believes the Defence leadership were scape-goating these soldiers to be seen to be doing something about war crimes when in reality they continued the cover-up for the murderers. This flauting of command responsibility is the bigger story which the abc continues to ignore.
Edit: also, motive was never discussed during trial. Trial only ever got as far as pre-trial, where the justice ruled on the meaning of 'duty' (just follow your orders) and in a closed session allowed the govt to scoop away David's evidence, leading him to plead guilty.