The death penalty should be given to any person who, without any doubt, committed a murder. This means that the death penalty could only be issued in cases where conclusive and irrefutable evidence links the suspect to the murder. Knowingly infecting others with an incurable disease (without the victim’s knowledge, such as in consensual sex) would also be classified as murder.
Acceptable proofs would include evidences that are as conclusive as or more conclusive than finding DNA that matches the suspect to the murder, clear photo or video images, the suspect’s revealing of information that only the murderer would know, etc.
Examples of unacceptable lines of evidence include testimony or eyewitness accounts from any other person or people, confessions by the suspect without revealing information that only the murderer would know, etc.
Using the above standards, the death penalty should be applied not only in cases of premeditated murder, but also in cases where recklessness or criminal negligence is found to have directly caused the death of another human being, even the death of a bystander who is not directly involved in the crime (who is neither a perpetrator nor an intended victim).
Justice systems which do allow the death penalty, but set lower requirements for its imposition, should immediately refrain from executing any person whose death penalty sentence did not derive from such conclusive lines of evidence as listed above.
The only exception to this rule would be if the criminal who killed did so as way to defend himself from future abuse by the victim. There should be some evidence that this abuse has occurred and that it is likely to occur in the future.
All convicted murderers should be required to pay all costs associated with their crime before being executed (although they could also factor in the prices they would likely receive by selling their body parts after being executed). Even though a criminal is far more likely to suffer a natural death before all bills are paid, and thus before becoming eligible for immediate execution, execution must not occur until all bills are paid.
Death Penalty for Extra Long Prison Sentences
There should be no such things as life sentences, multiple life sentences, or 400 year sentences. Perhaps 100 years should be the maximum prison sentence and anything over that amount should automatically require the death penalty. Most crimes should be denominated in terms of monetary fines anyway (reducing the amount of prison time a criminal is likely to rack up). For crimes which do require cumulative prison sentences of over 100 years, the criminal must first be required to work to pay off all the debts incurred, including the costs of incarceration, and then he may be executed.
Executions should occur in the least expensive and most humane way, namely, putting someone into permanent sleep using a gas, such as carbon monoxide. Even shooting by firing squad would be more humane than the electric chair or lethal injection.