Can German U-boat attacks on the Lusitania and sussex be justified?

Anything can be justified. A television program some years ago indicated (showed) a passenger roster showing a passenger possessed a box (20 cartridges) of rifle ammunition for sport hunting. That justified the attack and sinking of a civilian ocean liner (Lusitania in this case); the ship was carrying war supplies!

There's the letter of the law & there's the spirit of the law.