Absolutely but in extremes of cases maybe it is justified.
He was a mass murderer so in this case IMHO it is justified.
It was necessary, because he was not going to see punishment otherwise.
In a just society, he’d be in jail forever. Once again, Fuck the Capital Punishment, even in extreme Cases.
I don’t think capital punishment (murdering a helpless person in custody) is ever justified. It’s just cruelty for the sake of cruelty.
What we have here could be an act of societal self-defense, where the target was in the process of actively harming millions of people, and the legal system wasn’t doing anything to stop it. Whether or not it was self-defense or just a pre-paid hit for some other reason I can’t say, and neither can I judge whether it was justified or not. I just think it’s categorically different from capital punishment.
