That's an interesting question.

If you restrict "murder" to noncombatants, i.e., civilian passengers and crews, the answer may be none.

International naval practice held that merchant vessels could be seized or destroyed only after the passengers, crew, and ship's papers had been placed in a position of safety. That generally meant they were placed in lifeboats with provisions, or even taken on board the aggressor until she could reach a civilized port.

The use of the submarine in World War I changed that.