why is opelia's death so tragic? how/why does the queen know so many details about it?

Poor Ophelia was used and abused by so many people. Hamlet used her to enact his revenge plans. She was also collateral damage in Hamlet's vendetta against all woman because of his mummy issues. Her father uses her to get information about Hamlet; the King and Queen were also complacent in Polonius's schemes. I would think that Gertrude actually walked down to the gardens and say Ophelia lying in the creek.