Factual error: After Samuel shoots himself he is lying on the floor next to the Glock he used. The slide is locked back even though he had only one round and no empty magazine in the gun. The slide will not lock back in this configuration.
Factual error: In the scene where they are first being chased (and where Pita writes down the license plate number), the plate on the other car has the number C5T K89. All DF plates for private cars and trucks have the format 123 ABC, as seen on the car driven by Creasy (845 PHY).
Answer: Creasy was a contract assassin for the CIA. His downfall was his drinking because it affected his reaction time and so forth. He had a lot of bad memories he was trying to live with. He considered this bodyguard assignment to be a blow off assignment. This and his bad memories combined with his drinking problem, made him severely depressed. This is why he considered killing himself. At the end of the movie, you see Creasy being taken away in a car. His gunshot wound from the "Voice's" brother, along with his prior gunshot wounds that were never properly treated, had finally gotten the best of him. He was bleeding the whole last half of the movie because he never fully recovered from the gunshot wounds. So, to answer the question of why he died, he bled to death.