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.
The notion that prison is only for “housing criminals” is just wrong, and leads to the fucked up legal system you see in the US. Its primary purpose should be reforming someone to become a better person, drop their old ways and rejoin society. The percentage of people who can’t be reformed is vanishingly small if you do it right. And even in those cases, murdering someone just because they require resources to live is wrong. They are still human beings, however fucked up their actions, and deserve life and dignity simply because of that.
No, the main difference is the active and actual threat the person poses. When they are isolated in prison already, they do not pose any further threat.