Maniac Cop 2

After being impaled by a pipe and plunging into a river at the end of the previous film, the undead Maniac Cop Officer Matthew Cordell acquires a junked police cruiser and continues his killing spree through New York City. Finding a convenience store in the middle of a robbery, he kills the clerk; the thief is subsequently killed in a shootout with police. As Cordell stalks the streets, his enemies Officers Jack Forrest and Theresa Mallory are put back on duty by Deputy Commissioner Edward Doyle, who has the two undergo a psychiatric evaluation under Officer Susan Riley. While Jack is content that Cordell is long gone and wants to go on with his life, Theresa is convinced that Cordell is still alive and plotting his revenge.
At a newsstand, Jack is stabbed through the neck by Cordell, which leaves Theresa distraught and prompts her to appear on a talk show to inform the public about Cordell, as the police have kept Cordell's supposed return covered up, as Commissioner Doyle was involved in originally framing Cordell and sending him to Sing Sing. While en route to a hotel in a taxi, Theresa is joined by Susan, and the two are attacked by Cordell, who kills the cabbie and forces Susan and Theresa off the road. After handcuffing Susan to the steering wheel of a car and sending her into the busy streets, Cordell kills Theresa by snapping her neck. Gaining control of the car, Susan crashes and is found and given medical attention.
Elsewhere, a stripper named Cheryl is attacked in her apartment by Steven Turkell, who has strangled at least six other exotic dancers. As Turkell brutalizes Cheryl, Cordell arrives, disposes of a pair of officers earlier called by Cheryl, and helps Turkell escape. Grateful for the help, Turkell befriends Cordell and takes him back to his apartment, where Cordell stays for a short while. After Cordell leaves, Turkell goes out to find another victim but is identified at a strip club by Cheryl. He is arrested and placed in a holding cell by Susan and Detective Lieutenant Sean McKinney.
Turkell taunts Susan, telling him Cordell will break him out. Turkell's assumption proves correct, as Cordell breaks into the police station and massacres the bulk of the police force in a hail of gunfire. Using Susan as a hostage, Turkell, Cordell, and another criminal named Joseph Blum hijack a prison bus and head to Sing Sing, where Turkell believes Cordell wants to free all the inmates and create an army of criminals. McKinney and Doyle follow, and McKinney convinces Doyle to reopen Cordell's case and rebury his casket with full honors on the assumption that this will appease Cordell.
Cordell bluffs his way into the prison using Blum's paperwork, and the others kill a guard for his keys. Shortly after entering death row, Cordell is contacted over the prison PA system by Doyle, who admits to Cordell that he was set up and states that his case has been reopened. After hearing Doyle's announcement, Cordell abandons Turkell, Blum, and Susan and heads deeper into the prison, where he is attacked with a Molotov cocktail by the three inmates who originally mutilated him. While burning, Cordell kills the three convicts and assaults the other prisoners, only to be attacked by Turkell, who realizes Cordell used him. As Cordell and Turkell fight, the two crash through a wall, fall onto the bus below, and seemingly die when the vehicle explodes.
Sometime later, Cordell is buried with full honors alongside other fallen officers; Susan and McKinney attend his funeral. As Cordell's casket is lowered, McKinney throws Cordell's badge into the grave, leaves with Susan, and delivers a monologue about how there is a little bit of Cordell in every officer, and that every member of the force needs to rise above becoming a Maniac Cop. Before the credits roll, Cordell's hand bursts through the lid of his casket and grabs his badge.

Question: Why did Cordell abandon Turkell, Blum, and Susan?
Answer: Cordell found out he was setup and his case was reopened.