death penalty

I've been thinking about the imminent death of Nguyen Tuong Van and the whole debate around it. It seems that I'm in the minority in that I actually believe in the death penalty. (Please read to the end to see my unexpected conclusion to this arguement.) Now before you crucify me for the uncommon belief let me make an arguement.
I believe that the bible teaches that the penalty for sin is death. In the story of the women caught in adultery (found in John 8) Jesus does not dispute the fact that the women was rightfully condemned to death as it is the law of the bible. (see Leviticus 20:10).
So if someone is caught in sin, in Nguyen case drug trafficing I cannot argue that the death penalty is not valid.
Now you say in the case of the women caught in adultery Jesus saved the women, surely that means he was against the death penalty?
Well to argue the point let me give an analogy.
If someone were to kill someones child then I believe that the parent has the right to insist on the death penalty for the biblical punishment for murder is to be killed.
However, as a Christian and a believer in grace, I would hope that if it was my child that I would extend grace and mercy and not insist on the death penalty.
So let me summarise. I believe the death penalty is valid, but if I had the option of imposing the death penalty then I wouldn't, not because it wasn't valid, but because God's grace extended to me means that I need to extend that grace to others.
In the eyes of God I'm guilty and deserve to be condemned, but he has by grace extended to me forgiveness, his giving forgiveness does not mean the death penalty for sin is wrong, it just means it has not been exercised.
I'm not sure I've made the greatest arguement, but there it is anyway, my vent on the whole debate.
I'm of course willing to have my mind changed, but to change my mind requires me to be convinced that my understanding of God's teaching is wrong. In the end I'm accountable to uphold the truth of God not the current trend of human teaching.