Depends on the severity of the crime. If for instance, the mere act of keeping a person alive contributes to the deaths of others, then perhaps the death penalty is the only way to prevent more people from dying. In that case, the greater good would mandate that the death penalty be the only viable option.
In most cases, life imprisonment would be the best choice. There are other factors to consider, though. Prison space, murdering someone while already in prison, etc. I mean you can't make a blanket statement that says: "Never use the death penalty, ever. I don't like it, so no". That's just as irresponsible as someone saying the death penalty should always be used against serial killers, or pre-meditated murder. There's so easy answer, since it's not an easy topic.
Just something to consider: In other places in the world, the severity for small crimes like petty theft, etc, people are jailed, or even have a finger cut off. In those places, the crime rate is extremely low. Food for thought.