I agree. Most murders are the consequence of domestic crises and the perpetrators are not, usually, a threat to the general public. I have even heard one former prisoner claim that (largely for this reason) he preferred to share a cell with a murderer than with a hardened, professional criminal.
I would regard anyone guilty of a long, continuing series of rapes, violent assaults or child molestations as far more of a threat to society (and deserving a far longer jail sentence) than someone guilty of a domestic murder, notwithstanding the gravity of any such murder.
But there is a small number of killers (usually the ones who make the headlines) whose guilt is either admitted or beyond any doubt, who are completley unrepentant and whose crimes are so horrific that it is clear that - both in terms of 'justice' and in order to protect the public - they should never be released. And I strongly resent huge amounts of public money being spent keeping these people alive, safe and healthy - far more than is ever normally spent on any innocent member of society.
In such irredeemable cases, I would, like Sinick, have no qualms about pressing the button, any more than I would over slaughtering a sick animal or shooting a mad dog. You don't kill such creatures out of hatred or revenge, but because doing so is more humane than keeping them alive.
Locking people up and just waiting for them eventually to oblige you by dying of old age is far more uncivilised and is a cowardly cop-out for the squeamish.