I personally am against the death penalty. Last year I wrote an essay on my opinion of the death penalty, and found some interesting facts, such as the death penalty costs more than life in prison.
Anyhow, I think the death penalty should not be used since though judicial errors an innocent victim could be killed for someone else's crime(s), and that would be horrible. On this point, people who don't have money for a lawyer get public defenders, so their representation is sub par, possibly leading to a conviction.
As well, I'm Catholic, and one of the ten commandments is "Thou Shall Not Kill", this does not give any leeway as to who you may or may not kill, even if the person was a serial killer, killing them is still a sin.