Lusitania:
Captain Schweiger of U20 would have fired a torpedo into anything that floated past with a British flag on it. He was that sort of person.

Captain Turner of Lusitania should have been steaming faster or zig-zagging to avoid submarines. But he thought he knew better.
I don't think that the failure of the Admiralty to inform Lusitania of submarine activity was significant. I don't believe it was standard practice at that time.

The German Government had warned that ships flying the British flag were liable to be attacked. Lusitania was on a list of ships that could be taken up by the Admiralty as Armed Merchant Cruisers, but this was never done (she was too expensive to run).

As an unarmed merchant ship, the attacking U-Boat captain should have warned the crew and passengers to leave before sinking the ship.
In actual fact Lusitania was carrying war materials (artillery shells and fuses) which would have made her a valid target anyway.
People do love a conspiracy, though. Often the simple answers are not enough for some people, who see every action or inaction as significant and don't seem to realise that sometimes things just go wrong.

My own view is that if Captain Turner had kept up his speed and/or zigzagged, Lusitania would never have been hit. How much was his fault and how much the owners, we'll never know. Fast liners made many voyages across the Atlantic during WW2 unescorted without problems.