Death is a permanent state. A human has no right to put another person in a permanent state. No human is above anyone else, no matter how much better their ethics and morals are. Everyone essentially holds the same amount of good and evil inside them.
Besides, death sentence is not a deterrent. It's a provocation. Humans naturally try to survive (unless their psychic state drops so low that they don't want to live anymore). Being threatened with a death sentence is like a taunt. "I will not die, I refuse to die!"
Additionally, many people who may lose their will in life just go on a massive rampage so they get immortalized before they're arrested, judged, put to a prison and soon killed with some acid liquid while people are looking at him from behind a glass.
Look at Norway's prisons, then compare that to their crime rate.
I admit that it's sad. I feel especially bad hearing about what happened to this VA, even though I watched it dubbed in finnish when I was a kid. It's cruel and disgusting, but what can one do? What is done is done.