it is not nearly as egregious as hiring an assassin

Yes it is. The intent was to put that household under lethal threat. It wasn't about disruption or inconvenience - you can call a pizza out were that the case; it wasn't accidental, like failing to extinguish a cigarette oppose to dousing petrol - the call was obviously deliberate; it wasn't some mindless action - it was premeditated from the COD argument, and it's something the perpetrator has done many times before.

That it's only resulted in one death out of the apparently 100+ calls is an unremarked commendation for the police

You might as well argue that employing an incompetent assassin means you're less guilty than employing a competent one. Doesn't follow

The police aren't assassins, but they work with extreme violence as a day to day part of their jobs, and when they make mistakes people get hurt. Had the call been to delivery radioactive material, or explosives, or a cage full of tigers or something, eventually someone is going to die - the intent is otherwise the same

He should get life in prison asif he hired an assassin, and I hope his sentence dissuades other people from such callous behaviour and thought