J.K. Rowling has said that Snape is the only Death Eater able to produce a patronus charm. From the books we learn that this is due to his memories of Lily. It is presumable that the other Death Eaters, Voldemort included, are incapable of remembering a moment of joy (Even if they had such moments in their pasts). Their anger, bitterness and possibly guilt and self-hating, tainted any happy memories they might have drawn on.

In a scene from Harry Potter and the Deathly Hallows, Dolores Umbridge has clearly cast a cat-form patronus. Think about that for a moment. While brutally sentencing innocents to a lifetime of torture in Azkaban prison; while wearing the horcrux locket, which twists the mind and fills it with the darkest thoughts; when consumed, influenced, and surrounded by evil, Umbridge was able to produce a patronus charm.

She isn't evil because she is angry or bitter, she is evil because being evil, thinking terrible thoughts, and committing horrifying acts brings a warm fuzzy glow to her heart. Evil is what makes her happy.