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.
Other mistake: When Creasy thumbs through Pita's journal (which, unusual for American English, is addressed by her as "Dear Diary"), you can catch a glimpse on her first entry, made on a page titled "January 1", but reading "Today at school I had a great time." So, if she's alleged to be keeping her diary true-to-date, she's describing a school day on New Year's Day. Also, her handwriting changes considerably within the next three months (cf. April entry) - but then suddenly changes back in June.
Character mistake: The mother is supposed to be American, but she speaks in her native Australian accent near the end when she says, "I don't think I can take anymore of this" upon finding out that Pita may be alive.
Character mistake: When Jordan Kalfus is reading the instructions off the paper, the word divided is spelled 'devided'. (00:59:52)
Continuity mistake: When Creasy is repeatedly shot, in one shot there's blood on the car next to him, but in the next shot there isn't.
Other mistake: In the cemetery all of the headstones are blank except for Ryer. (01:26:25)
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.