If you did something that would win you a place on death row then you fully deserve to die. Keeping them alive with a life sentence does nothing but drain money and take a spot in our prisons, which are crowded as it is.
Again, the death penalty costs more.
Besides, the prisons aren't full because of murderers, they're full from non-violent drug offenders. If the private prisons didn't profit so much off of the large inmate population, drug offenses would be met with rehabilitation instead of incarceration. Then the prisons would have room for the people that actually deserve it.
And if you ask me, the death penalty isn't a punishment. Rotting in prison until you die of old age is.