Most crimes aren't premeditated so it would only be a possible deterrent in a small number of cases. Life without parole would fulfil the same function as most of the pro cp arguments.
If there's any merit in a pro capital punishment argument, I think it's only that the death penalty might bring a sense of closure to the victim's family in a way that imprisonment doesn't. They wouldn't have to endure the likes Ian Brady having years of notoriety and news coverage whilst jailed for example, continually reopening the wound.
It would be a completely unworkable system though to base capital punishment on the wishes of the family obviously, and in some cases, like Shipman, families seem to feel that he managed to escape responsibility.
We have the money and the knowledge to improve the criminal justice system, including dealing with the most horrific of offenders, there just isn't the will to do it.