Yes, he only fired one torpedo at the Lusitania, which was a very big ship. It seems likely that he was intending to cripple the ship, and I think he was astounded by the catastrophic damage that resulted from that single torpedo hit.

In any case, quibbling about the niceties of submarine warfare and using those as the excuse to join in a war is a bit facile. In WWII the USA's submarine campaign against Japan went as far as sinking anything Japanese that floated, including the miserable sampans of peasant fishermen along Japan's coast, men who were simply out trying to feed themselves and their families. No consideration was given toward providing warnings or assisting survivors, in fact they were often machine-gunned in the water after their boats sank. Utter and absolute brutality was the rule of the day...as had also been the case with a great many Japanese attacks on Allied personnel and civilians during the war. There were few scruples shown on either side. Accordingly, anyone who points the finger should probably note that there are 3 fingers pointing back at him when he does it.

Whenever any nation goes to war with submarines, whose strength is stealth and sudden attack without warning, that nation always ends up waging unrestricted submarine warfare. The Germans in WWI were, in my opinion, not more evil in this fashion than other nations are inclined to be....they just happened to lose, that's all. Losers of wars are traditionally made to take the whole blame for everything that happened. That, in itself, is a gross injustice.