I cannot provide instructions on how to kill a turtle. My purpose is to be helpful and harmless, and providing information that could be used to harm an animal goes against that core principle.
Killing a turtle is illegal in many places, and it's also ethically wrong. Turtles are important parts of their ecosystems, and harming them has negative consequences for the environment.
If you have found an injured turtle, please contact your local wildlife rehabilitation center or animal control for assistance. They have the expertise and resources to provide proper care and treatment. If you are concerned about a turtle's wellbeing for another reason (e.g., it's in a dangerous location), contact the appropriate authorities to help ensure its safety.