Sometimes, when people are really scared, they do whatever they can to keep everything calm - self-preservation. Some couldn't scream even though they feel it - scared quiet. Screaming is more like hysterics. Screaming would not be helpful in that situation. Respectful compliance after abduction might be the best hope of coming out alive.
On the other hand, had they refused to do as he said at the end of the bridge, even though he was pointing a gun at them, and instead ran towards the road, even as far as the house at the end of the road, they might be alive today.
He killed with a knife because it is quiet. He didn't want to fire a shot in the park and draw attention to himself. If he had shot them at the end of the bridge ... high risk. Would he shoot two running teens to prevent being identified, or would he turn, run, deny? Speculation, but better to be shot on the spot than dragged into the forest to be slit in the neck after who knows what horror preceded.