Timing, Peter, timing!

Explanation from original meme maker:

Perter White was a Lieutenant in the British Army in WW2. He describes in his memoire how after capturing a town they captured some German soldier and their weapons. These German prisoners were gradually being taken back to the rear by his men when he decided to try out some of the captured weapons.

From the perspective of the captured Germans they had just seen their friends get taken away followed by shooting from outside this house they were in. When White reentered the building he found the Germans terrified believing they were about to be executed.