Having a comparator does not automatically make something light.
Water torture is not “light” simply because we’re not gouging eyeballs and cutting off testicles. Burning someone with acid is not “light” simply because we’re not actively lighting them on fire.
You have yet to provide any justification for your claim that imprisonment is “light” other than that it’s not death. You can’t justify barbarism simply by saying that something else that isn’t barbarism is lighter by comparison, and therefore barbarism must be justified. Were that true, you could try to justify any proposed barbaric act by saying that the second worst thing is “light” by comparison.
What is the necessity of killing someone after 36 years of not killing them? There’s clearly not a safety concern, or a concern of escape, or anything else pressing. It’s so far removed from the original crime that it’s not really a punishment for that crime anymore: the last 36 years of imprisonment were the punishment. It’s just an act of barbarism for the sake of ticking a box.