I don't think we should kill anyone. At the very worst, we should put them in solitary confinement, and I don't even think we should do that unless it is necessary for the safety of the individual. We should do our best to make sure everyone has what they need to be mentally and physically healthy and relatively happy, including in prison (I believe in rehabilitation, not punishment.).
Some people aren't born, they're created, and I think very few terrorists are 'born' per se: you don't become a terrorist because you're a normal person, you do it because you're a freaking lunatic, and while you're responsible for your actions, elements of society are also somewhat responsible for having produced you. It's regrettable that Taernav became a terrorist, but I don't think we should take away his ability to be alive, or to be happy, or treated with kindness, just because he killed quite a lot of people and caused severe negative impacts on dozens of others.
Yes, that is bad. Yes, it is his fault. And yes, he has to deal with it -- and so do we. But I don't think it's right to lose all sympathy for someone, or to stop seeing them as a human being, just because they commit an atrocious and awful act of violence. He's not innocent, but he's not a literal monster. His brain and his body are still very much human.
As a result of this, I think we should treat him with a measured level of mercy. Measured for the safety of the other people around him.
Obviously I want serious prison reform, but I'd settle for at least not executing anyone, including mass murderers and terrorists.