[MainFrame65]

I believe he was still thinking of himself as a righteous hero as he walked away from the truck. The realization of his monstrous crime, and the shame of having been used by the enemy, came later. And he chose to die in silence, as quickly and painlessly as possible, rather than to admit how badly he had failed his Country, tell the whole story, and help find the plotters. He played the brave, silent soldier, but actually died a coward, unable to own up to his failure.

[Wolfstar]

The only thing I can't quite believe is that he was ever ashamed of what he did. Of course, we'll never know. But I think if he did experience true remorse, he'd tell somebody the truth of what happened. Instead, as you so rightly call it, he chose the coward's way out.