That is completely not true. They have a life, they can walk, talk, read books, and enjoy what little prison has to offer. We are all waiting for death, they must wait in a place that they can not harm others. Whether they are in prison or on the streets, Father Time will do his job.
You said the only reason you support the death penalty is because "rabid dogs" should not participate in or harm society. Life in prison is an easy way to do this, yet you support the death penalty instead. There is no reason to support the death penalty other than vengeance. On both fronts, economically and morally, life in prison is far more acceptable than the death penalty. So those that have no moral qualms and are simply looking at the numbers should support life in prison if they are being objective. Those that do not support the death penalty on moral grounds are well within reasoning.
So I ask you again, why do you support the death penalty? You said its not for cost reasons, so utility goes out the window. You said its so these people won't harm others again, prison does that. So what is it other than vengeance? If it is vengeance and anger, just say it. Those are human emotions.