In Blade Runner, how does Deckard show pessimism?

The lack of hope in the world and his place in it represents Deckard's pessimism in Blade Runner.
Deckard's lack of optimism is seen in his life and the world around him. His pessimism is evident his job:  "They don't advertise for killers in the newspaper. That was my profession. Ex-cop. Ex-blade runner. Ex-killer." Deckard does not use any rationalization to justify what he does. He understands his function in the world is one of negation: "The report read 'Routine retirement of a replicant.'  That didn't make me feel any better about shooting a woman in the back."  
Deckard's negative tone is a result of the world in which he lives.   It is a setting where human emotions can be simulated emotionally cushioned" and simulated, corporations can do whatever they wish without any form of governmental limitation, and the police have the authority to kill anyone deemed a replicant. The film's cinematography enhances this through its use of dark and shadows, conveying something sinister and unsettling about the world of the future.
Deckard is unable to convey restoration and hope because the world in which he lives lacks these elements.  He cannot be a force of redemption in changing the world for the better.  Rather, he can only escape it.  Deckard's pessimism is rooted in a world that lacks hope.