The thing about an effective justice system is the rules need to be the same for absolutely everyone. If that becomes a problem, then it's the system itself that needs to change. But what they absolutely should not do is build a system, and then make exceptions on a case-by-case basis. If you make exceptions to rules about how criminals are treated, even if it's because one criminal clearly deserves something more than another, it undermines the whole concept of having a system in the first place.
Imagine applying this same logic to game design. Every enemy has either armor or a shield. Armored enemies can only be damaged by ranged attacks (let's say arrows), and shielded enemies can only be damaged by melee attacks (swords). If, after a few hours of game, you introduce an armored enemy who can only be damaged by melee attacks and justify it because "his name is Archer Destroyer, off course arrows can't beat him", even if that justification seems reasonable, it completely eliminates the entire purpose of having a consistent system up to this point.
Now I know this isn't a perfect analogy, and I'm not saying this guy deserved anything but a death penalty in actual human terms. Hell, I'm largely against the death penalty, but I've always felt there should be exceptions for people with a high chance of repeating their crimes. But such exceptions need to be defined in the legal system for whatever country they're in. If you don't define such allowances, but then grant them anyway, it sends the message to the people that the courts can do whatever they want, and the laws don't matter. For the rulemakers not to be bound by their own rules, and able to make any decision they want unchecked like that, no matter how right that decision may be, is borderline despotic behavior and a very bad path to start down.
tl;dr - If he is getting legally-guaranteed rights that he should not get, the solution is to change the law, not ignore it.