Use of the death penalty as bargaining leverage is unethical for prosecutors IMO, and it is problematic in other related ways:
Coercion and Undue Influence:
Threatening a defendant with the death penalty, especially when the prosecution may lack the resources to secure a death sentence, can exert undue pressure on the defendant to waive their right to a trial and plead guilty, saving the prosecutor's budget.
Erosion of Constitutional Rights:
This practice can undermine a defendant's right to a fair trial and due process, forcing them to choose between facing a potentially unfair trial with the risk of death or accepting a plea deal that may not accurately reflect their guilt. "Am I risking death?" is a much different question than "Can I get a lower term in prison".
Potential for Wrongful Convictions:
The pressure to accept a plea deal can lead innocent individuals to plead guilty to avoid the risk of execution, resulting in wrongful convictions.
Ethical Concerns:
Some argue that it is inherently unethical to use the threat of death as a tool to secure a conviction, regardless of the circumstances.