i know i'm asking a lot of questions, BUT who killed Fred, how did he die?

A death Eater named Rookwood had shot a killing curse aiming for Percy but Percy ducked and it hit the wall of Hogwarts and there was an explosion and Fred died

I don't think we know who killed him, but we know how: he died in an explosion during the battle of hogwarts; a death eater cast a spell which caused the ceiling to explode.

After Harry, Ron, and Hermione had destroyed the diadem, they ran into Fred and Percy in the halls. They were fighting Death Eaters. Percy was newly reconciled with the family and was cracking jokes along with Fred. Fred was laughing and commenting that he hadn't heard Percy laugh or joke in ages. There was an explosion and the side of the castle was blown away. Hermione and Harry were lifted and blasted into one direction. When they recovered they heard screaming. Ron, Fred, and Percy were against another wall. Percy had realized the explosion had killed Fred. He stayed trying to protect his dead brother and had to be pulled away to continue fighting. It was never stated who caused the explosion.

NOOOOO... FRED COME BACK! Dammit... I didn't need that!

Rookwood shot a spell but it missed and hit the wall behind them and then the wall fall so rookwood

Fred saved percy
as telling a joke right after this
bellatris lestrange kills him
"his last laugh etched across his face"