Which incarceration already accomplishes.
No it doesn't, unless you believe that prison guards, prison staff, and other prisoners who are in the process of being rehabilitated are not part of our society. The expectation with converting all death penalty cases to life in prison is that we're going to take the most violent, dangerous, destructive people in the world, put them all in one place, and expect a segment of society to wait on them hand and foot for the rest of their lives. Anyone wishing to make that happen should sign up for guard duty with those prisoners. Otherwise, hypocrisy.
They aren't "permanently removed from society" just because you, personally, don't have to interact with them on a daily basis. Someone else still does and you're putting their life (in fact, thousands of lives) at risk because of it.
Ethics of death penalty aside, pretending the choice is it or letting murderers walk free is dishonest.
Life in prison without parole is a death sentence; merely by different means. If we can't ever execute anyone because we can't be 100% certain of their guilt, then why is it morally or ethically sound to allow potentially innocent people to rot in prison for the rest of their lives? Or even just for a few decades? There's almost certainly more innocent people who've been rotting in prison for decades than innocent people who've been sent to Death Row. Not just nominally, but due to the higher burden of proof (and endless appeals system) for Death Row prisoners, I would suggest it's a virtual certainty that the rates of innocent people rotting in prison for the rest of their lives is also much higher. Why is that more acceptable than executions?
Does this mean that contract killers should get leniency because, after all, they're not killing people for revenge but as mere business? Especially if they use a clean method - which, as the summary noted, is what "humane" really means in this context?
They're committing murder. The state is not (by the very definition of murder). I would submit that all murderers should qualify for the death penalty. I would extend that out to any other individual who cannot be rehabilitated (assuming an effective means of rehabilitation is in place - which is admittedly not our current prison system).
And keep in mind, this is all merely a thought exercise as it's all predicated upon major, fundamental reforms from the police investigations to the courtroom trials to the prison system. I'm not defending the criminal justice system we have today as it's vastly more flawed than need be and I'm not even defending the use of the death penalty within that system. I seek merely to demonstrate that there is nothing inherently wrong or unjust about the state executing guilty individuals who are truly beyond rehabilitation.