The shortest answer is that violence is inherent to every political system, in fact, that is the sole purpose of politics, to determine acceptable violence in a society, and who controls it, and who doles it out.

This includes more extreme violence like killing.

Anyone who has seen the mistakes the state has made and will continue to make will be against the death penalty - not because of the violence, as I can guarantee you they support violence in other ways - but because of the accuracy. In a perfect world wherein we know there is no possible way to redeem someone and we can guarantee a perfect conviction, very, very few people would be against a state-led death penalty.

Now Luigi, and all vigilantism directed towards the top, or mass killers in general, is the same thing. It’s unpopular when it’s against unjustified targets that have a margin of error. Luigi’s target had no margin of error. He was directly responsible for hundreds of thousands of preventable deaths. It not only was public knowledge, he received money for this, he received prestige and recognition for his kills. So of course people cheered on the killer of the mass murderer. That’s natural.