War crimes?
Because there weren't enough lifeboats?
Even though the captain & other crew members went down with their ship?
I'd say, more likely simple incompetence increased by the stresses of wartime.
It reminds us, again, that Brits are want to take risks with safety when they think they might get away with it.
For a more recent example, you could consider all that oil spilling into the Gulf of Mexico...
But if you are really interested in a serious definition of the term "war crimes," you might take a gander at Poland, and see what the Nazis were up to there.
Article 46 of the Geneva Convention makes specific mention of this though:
"The Detaining Power shall supply prisoners of war during transfer with sufficient food and drinking water to keep them in good health, likewise with the necessary clothing, shelter and medical attention. The Detaining Power shall take adequate precautions especially in case of transport by sea or by air, to ensure their safety during transfer, and shall draw up a complete list of all transferred prisoners before their departure."
I can't think of a case where anyone was ever prosecuted for failing to properly mark a prison ship that was subsequently attacked though.
The ship should have been well lit, marked with a red cross, and properly identified as carrying prisoners. Instead, it was repainted grey, and may well have been an armed merchant cruiser. (One of the surviving crewmen is listed as Deck Hand [Naval Gunner}, http://www.bluestarline.org/arandora_crew_list.htm which supports the German communique in panel 9 that the ship was ‘an armed English steamer’. (BBC reports only ‘heavy machine guns’ for protection)
In any event they probably got better treatment on that ship than did the survivors when shipped out again, this time on the Dunera. http://www.aufrichtigs.com/01-Holocaust/Dunera/Robert_Aufrichtig_-_Dunera_Internee.htm