When the good thief said, hanging next to Christ, that his and his fellow criminal's executions were just, why did Christ not explain that the death penalty was wrong?  In point of fact, Christ rewarded St. Dismas with paradise, for accepting his justice and asking forgiveness.  Mercy can be found in the fact that, despite our sins, as long as we seek forgiveness we can still receive our eternal reward.  In the case of Capital Punishment, executing criminals humanely is an example of mercy.
Is Capital punishment the reason for high murder rates?  Hardly.  If the death penalty does not act as a deterrent it is because the convicted spends on average 13 years on death row.  If the sentence was carried out swiftly, it might well be a deterrent.  Murder is barbaric.  Its just punishment isn't.  By not carrying out justice the state devalues the lives of the victims.