Because by not having it, there are more innocent deaths, without a trial or options for appeal.
You can argue for full term tariff using the 'one death too many' but not simply against capital punishment, as it ignores the other innocent victims.
Full term tariff open up a host of other complications, as deaths in cells, or the feeling of nothing to lose before capture are among a few of the consequences.
Saying that, studies in mental institutions show that those with no hope of release seem resigned to their fate, and less likely to misbehave or abscond than those with a long release date.