I think at the end of the day the basic justification for killing Nero is "he's an evil man who did evil things and therefore he deserves to die".
In essence, it's the same question whether you support the death penalty or not. Is it justifiable to do to an "evil" man what he has done to others? Or is such an action also evil in itself?
And to say that Nero was maybe
still a threat and therefore it was necessary to kill him is just a rationalization for those who are getting uncomfortable facing that question.