I'm with you on this one Ricky.
I'm not squeamish, and if the armed police had shot these people dead, I wouldn't have turned a hair, and I don't give a monkeys if a householder kills a burglar they discover in their house.
But there is a difference between police (or an armed householder) killing in self defence or defence of others, and the state coldly and calculatingly killing someone, even lowlife scum like this.
I like to think that I am better than these people, my friends and neighbours are better than them, and the society I live in is better than them. If the price of proving that is that we spend money keeping them alive in jail, then thats a price I am prepared to pay.
I also remember several cases where supposed criminals were found guilty "beyond reasonable doubt" of what would be capital crimes, and subsequently turned out to be innocent. As you correctly point out, you can release someone from prison, but you can't un-execute them.