Zaheer can't be blamed for Kuvira's actions. He created the opportunity for her to take power, but SHE is the one who took power and did all the things she did so she bears the responsibility for them. Trying to invoke the chain of causality is stupid because taken to it's logical conclusion, you'll have to put parents in jail for crimes their kid did since the kid would have never existed without the parents. Ultimately, we are responsible for our actions as individuals and while Zaheer can be blamed for creating the situation he did, he can't be held responsible for Kuvira.
As far as his sentencing goes, he should have been killed. Not because it's just or 'what you gotta do' as vas up there says. Just because nothing else makes sense. When your dealing with a terrorist of Zaheer's magnitude, true justice is impossible. Like, where are you going to find a jury of unbiased peers in the earth kingdom, whose entire infer-structure he's annihilated? What judge is going to be fair? What defense lawyer is going to put in the slightest effort into seriously defending him? It'd be like if we were to seriously put Hitler on trial. Everyone would enter it with their minds already made up, before they heard the first piece of evidence. THe whole thing would be a farce due to his notoriety no matter what.
Even if he somehow avoided a summary execution, and got a 'fair' trial that sentenced him for life or whatever, he'd realistically be shanked at first opportunity by the prisoners he's sharing the prison with. Or, if he's in such complete isolation, then the guards would have a 'slip up' where they claim that he tried to escape and they had to defend themselves. And no one would care enough to conduct a serious investigation checking otherwise.
Congratulations, Zaheer. You wanted an environment where people can do what they want and do not allow themselves to be binded to any authority. Well, you got it. As Guru Lahima once said, "Don't drop the soap."