I don't think you guys realize that eternal solitude is worse than death.
you can understand it and still disagree with it. rationally I would argue killing him would have been the better choice. The way it is now he could figure out a way or get someone to help him lift that curse and be a nuisance again.
Kinda like with Batman and the Joker. The Joker causes so much death and destruction but Batman just keeps throwing him into Arkham Asylum just for him to break out again and kill some more. Obviously for story reasons it has to be that way, but just through a utilitarian lens at some point someone should've killed the guy, be it Batman or the death sentence through the justice system. (and I say that as someone against the death penalty IRL, but super villains are a different story)