Despite the good guys believing he would survive, something went wrong during surgery, and he passed away in the hospital shortly before Sara arrived from Legends of Tomorrow for a goodbye.
The final battle against Ricardo Diaz in Arrow Season 6 was not without consequences for Team Arrow, and one result was more dire than Oliver winding up in prison.
Quentin Lance was shot in an effort to save Earth-2 Laurel Lance from Diaz, and although he was successful in preventing her death, his gunshot wound proved fatal.