You definitely got the tougher side to argue. You'll have to be prepared to address all of the strongest arguments against the death penalty. Among them:
1. By far the most compelling is this: Sometimes the legal system gets it wrong. In the last 35 years in the U.S., 130 people have been released from death row because they were exonerated by DNA evidence. These are ALL people who were found guilty “beyond a reasonable doubt.” Unfortunately, DNA evidence is not available in most cases. So, as long as the death penalty is in place, you are pretty much GUARANTEED to occasionally execute an innocent person.
2. Cost: Because of higher pre-trial expenses, longer trials, jury sequestration, extra expenses associated with prosecuting & defending a DP case, and the appeals process (which is necessary - see reason #1), it costs taxpayers MUCH more to execute prisoners than to imprison them for life.
3. It is not a deterrent. Violent crime rates are actually HIGHER in death penalty jurisdictions. This may seem counterintuitive, and there are many theories about why this is (Ted Bundy saw it as a challenge, so he chose Florida – the most active execution state at the time – to carry out his final murder spree). It is probably due, at least in part, to the high cost (see #2), which drains resources from police departments, drug treatment programs, education, and other government services that help prevent crime. Personally, I think it also has to do with the hypocrisy of taking a stand against murder…by killing people. The government fosters a culture of violence by saying, ‘do as I say, not as I do.’
4. There’s also an argument to be made that death is too good for the worst criminals. Let them wake up and go to bed every day of their lives in a prison cell, and think about the freedom they DON’T have, until they rot of old age. When Ted Bundy was finally arrested in 1978, he told the police officer, “I wish you had killed me.” Khalid Shaikh Mohammed (the architect of the 9/11 attacks) would love nothing better than to be put to death. In his words, "I have been looking to be a martyr [for a] long time."
I hope that helps - good luck in your debate!