In practice, the death penalty has killed some who were later found to be innocent, and it's been applied cruelly and selectively at times. It's not much of a deterrent (or at least no more so than life in prison), it's costly, and it's unnecessary. If having it in it's current form or abolishing it completely were the only options and I could snap my fingers to settle it, it'd be gone.
That said, some people actually do deserve it.