As an aside in an otherwise unrelated talk, I heard a priest say the other day, "How can there be any logic in capital punishment? How can you teach people to respect life by threatening to kill them?"
Regardless of what one thinks about the legitimacy of capital punishment, this is a bad argument. Throughout history, legitimate authority has used the threat of legally sanctioned violence (punishment) to prevent people from committing crimes, and it does indeed work pretty well. Not only that, but there's an example from everyday life that most people have direct experience with: Telling young children that biting, kicking, scratching, hair pulling, kicking, hitting and any other physical attacks I haven't thought of at the moment will be met with a spanking actually works very well. Indeed, at the ages of 3-8 when children are capable of more-or-less controlling their actions but have very limited ability to empathize with others (especially others who are making them angry) it's often pretty much the only effective manner of preventing intra-sibling fights getting nasty.
And contrary to the similar claim that "you can't teach someone not to hit by threatening to hit them", many of us in fact learned that hitting was not an acceptable means of self expression by this very means, and in turn have taught our offspring the same way.
Like it or not, we experience codified punishments handed down by a recognized authority as different from ad hoc violence used to vent one's personal feelings of the moment. And while threat of punishment alone will not serve to make people actually value life or eschew violence, it is pretty effective at preventing the proscribed behavior.
Other arguments against capital punishment (whether practical or moral) are compelling to one extent or another, but this one should honestly be dropped. It just doesn't ring true.