Socially, I don't have much regard for either "justice" or "mercy". They're both features of a moral worldview based on concepts of good and evil that I don't ascribe to-- I see actions in terms of "good" and "bad", rather than "good" and "evil". It's generally bad when people's lives are taken from them, whether it's in a prison cell or in the execution chamber. It is generally good when order is maintained.
Thus, the issue is not whether to balance "justice" with "mercy", but rather to balance the certain harm done by a harsh criminal sentence against the potential harm of a lesser sentence allowing the criminal to re-offend.
I lean heavily on the rehabilitation side of the equation. I think the primary goal of the "corrections system" should be correction, in salvaging what is left of the offender and returning him to society as an upstanding citizen. It's only when rehabilitation is too risky, too unlikely, or has failed multiple times that we should resort to harsh measures-- and then, rather than damaging the offender further and then re-releasing him, we should simply put him down as humanely as possible to prevent any further damage.
On a more personal level, when given a choice between retribution and mercy... I will almost always choose retribution. I don't have the means to provide rehabilitation for those whom have wronged me, but I do have the means to make such a bloody example of them that they will never risk crossing me again and everyone else will think twice about it.
