Porque ele mudou de lado.
Jedburgh foi contratado para "lidar" com Craven, mas gostava dele e assim o deixou investigar por suas próprias razões.
Quanto ao senador ... ele estava tentando "girar" o incidente em um positivo e Jedburgh não concordou ...
Jedburgh, who is revealed to be suffering from a terminal illness, meets with Moore, the Senator (for whom he had been working) and the political advisor who assigned Jedburgh to eliminate Craven.
He listens to their suggestions as to how to play the Northmoor incident in a positive light. He tells them that he is done and then suggests an assassination attempt on the Senator should be the feature story, to drive Bennetts death out of the tabloids.
They are happy to go along with the story until Jedburgh tells the senator that he is on the wrong side of the equation. Jedburgh then pulls out his gun and shoots all three men dead.