There is an upside to executing murderers. It's called justice. That's the biggest reason for capital punishment. Not being a deterrence. Having a murderer do 25 years or so and then get released is a spit in the face to justice. And it's no sign of "humanity."
Simple: they don't get out after 25 years. That's it. Let them rot in jail until they're collected away by the reaper. Getting raped, abused and eventually turning mad. Many consider this punishment worst than instant, soft, painless death. Theoretically, I'd be willing to consider that. But, as we've seen both in this thread, and in real life, they often do get out after 25 years or however long.
And as much as opponents of the death penalty like to talk about how life in prison is a worse punishment, it's easy to say when you're not facing the needle. It's why most facing execution try to plead down to life in prison, as well as file endless appeals to escape death.
