Rather than sign off on an agreement with the Canadian government to take the death penalty off the table, the prosecution team in the U.S. took an interesting approach: they simply told the Canadian government to do whatever the hell they wanted with him.
This put the Canadians in an interesting bind. They had a guy on their hands who was facing no criminal charges of any kind in Canada, so they had no legal basis to keep him in custody. Without any formal charges in Canada, their only legal recourse was to turn a serial killer loose on the streets of Vancouver. If I remember correctly, they figured out a way to quietly hustle him out of the country to the U.S.
IIRC, this multimurderer was a cannibal-American from California, residing in Calgary. He was turned over to the USG within months.