The U-boats could be mercilessly: machine gunning survivors in the water
In the interest of balance, I'd check out the Laconia incident.
This is interesting because it was Sep of 42. By that time, there were many documented incidents of U-boats harming innocents: S.S. Caledonia and Essie Bolivar in March of 42 had life boats machine gunned; S.S. Oregon in Feb had men machine gunned in the water. Several other incidents along the east coast and in Caribbean. It may be a part of the reason for the American reaction.