They are war criminals!
There is discussion in several threads about changes from novel to film and why.
I my opinion (humble, of course), the many incremental changes result in a very basic difference in outcome. The consensus of this forum is that Brando did not wish to be a baddie so changes were made.
In the novel, Diestl the Brando character, starts out good but gradually changes into an evil man. He is determined to survive the war by any means. At the end he gets a bit greedy and decides he can kill 2 more americans before he deserts and hides. Just for the hell of it really. This doesn't come off and he is killed himself.
In the film, he smashes his gun and walks toward the americans, unarmed and without trying to conceal himself. He does not offer any threat.
The two americans do not behave as if threatened (they do not take cover). They are not in the heat of combat. He is casually shot dead without being called on to surrender.
Frankly, this is murder. It probably did happen that way at the time but violated the conventions of war and was not always condoned, as Captain Richard Winters of the Band of Brothers recalled in his memoir.