Interesting muawiyah. Give us additional information about German Naval personel beating to death and beheading belligerent merchantmen and sailors. This is the first time I’ve heard about it.
After a war one of the best sources of information concerning what the enemy did are interviews with survivors from the enemy's own military, particularly if they lost.
With respect to the submarine service there were so few survivors (10%) such interviews can't begin to cover what happened. That's why it's risky to try a comparison between the Japanese Navy (a large, modern and major force in the Pacific War) and the German Navy (a far smaller, mostly submarine based, insignificant force in the Atlantic War ~ not that they didn't nail a lot of Allied shipping early on, but they were eventually DESTROYED).
Now, do you want to talk about German machinegunners shooting Allied merchant marine in the water? I'd suggest their use of torpedoes was excellent and they really didn't leave all that many guys around to shoot. What say you?