I am against it.
It is more punishing for the person to actually spend all that time in prision, and therefore more effective.
The criminals that do reform get another chance where as if they were killed beforehand there is no point. (the jail system is meant to change the criminals actions)
The death penalty does not deter murders, as most murders are commited by people upon people they know. These people arnt thinking of the consequences at the time.
It costs more money to put someone to death than it does to leave them in prision for life.
And also the obligatory: killing the killer just makes a hypocrite.
[edit]
You're an idiot if you think that the Christian God is forgiving.
HURRRR
The basis of Christianity is God giving humans a second chance through the death of Christ.
As I have come to know it in my life "second chance" = "forgiveness".
Not to start any religious argument of course.