Why did the Germans sink the Lustitania?

Yes, i think the Germans did have the right to torpedo the Lusitania. Only because they thought the Lusitania was carrying arms to help the British. Yet they were right. It is terrible that 128 Americans and lots of british were on this ship, and were killed when they mostly didnt know the arms were even on the boat. the Germans took their chances by doing this, but it was for their country!