Is torture a valid alternative to the death penalty?

You're starting with the presumption that the death penalty is solely retributional. I don't care for that presumption. The death penalty serves other purposes in society, like permanently removing a menace who is incapable of rehabilitation. Now, if you wish to use this standard (permanently removing a menace incapable of rehabilitation), you can, but then there's the issue of whether US C rights are afforded and what not. We don't just hand the government unbridled power in this regard. But neither do we obstruct our military in time of war.