The Zero Requiem plan was evil, if it involved restoring Zero's reputation as a champion of justice. the evil Emperor Lelouch should have been killed by Suzaku disguised as an entirely new hero - Infinity, for example, and Zero's reputation should have remained tarnished.
Zero murdered his rivals for leadership of the Japanese resistance by killing many JLF fighters as well as Britannian soldiers with his landslide at Narita and blowing up the ship with the remaining JLF leaders. Zero came up with a flawed plan for the Black Rebellion since he was counting on the Emperor coming to meet him after Japan proclaimed independence, which would never happen, especially after Zero killed Euphemia which would certainly scare away the Emperor. Then Zero abandoned the rebellion to defeat by the counterattacking Britannians (which probably saved a lot of lives but could be considered desertion).
In the second season Britannia heavily defeated the Euro Universe and arranged a marriage contract between the First Prince and the Chinese Empress, which could lead to the union of two thirds of the human race. The rest of the world would have to negotiate to become protectorates or client states or something to avoid being conquered and turned into Areas. Then the world would be united for the first time and there would be peace for countless thousands or millions of years. But the harsh Britannian oppression of the Numbers would fade away in generations and centuries as the Numbers and the Britannians gradually merged into one new people.
And Zero could have sped up that process by assassinating or geassing into obedience enough members of the imperial families and the governments to have princess Euphemia proclaimed co-empress of Britannia-China. Euphie wasn't bossy and would go along with all of Oddysses's plans except that she would fight him over any attempt to retain any of the cruelty and oppression of the old system under Emperor Charles and would demand a kinder, gentler, Brittannia.
Oops, Zero couldn't do that because he had foolishly wasted Euphemia's tremendous potential for good for no apparent reason during the Black Rebellion.
Instead Zero blocked the Chinese-Britannian marriage and foiled Britannian attempts at world conquest. Once the exhaustion of the recent wars and the relief from the Zero Requiem wears off, wars would resume and keep up for countless thousands or millions of years, taking the lives of millions or billions or trillions of persons.
Zero lead an invasion to liberate Japan which resulted in the enemy using F.R.E.I.J.A and killing millions. Of course, if Zero hadn't murdered Euphemia Nina wouldn't have tried to make a F.R.E.I.J.A weapon and later joined the team which succeeded in developing F.R.E.I.J.A, helping them to do so sooner than otherwise and thus having it ready in time for the wars of the second season.
Zero ordered the Geass Directorate Massacre. He ordered the massacre of many thousands of people instead of using his geass on those of them who were dangerous to convert them into useful followers. Zero ordered the massacre of children. The only thing that made Zero a little bit better than Earth's Supreme Horror, Genghis Khan, or the Terrible Tamerlane, was that he only ordered one terrible massacre.
I say the world needs a totally new hero, not Zero. If the world accepts Zero as a noble hero and an example to follow standards of conduct are bound to fall.
I have read that as part of the Zero Requiem Emperor Lelouch confessed that he was really guilty of the Fuji Massacre and used mind control to make Euphemia order the massacre. So that cleared her name and people no longer thought of her as the Massacre Princess.
But if Lelouch wanted Zero to become a hero again he probably left out important details, such as he had been Zero at that time he made Euphemia order the massacre. Such as Euphemia's struggle against the geass command. Such as the way that Euphemia seemed to break free of the geass command while she was talking to Lelouch/Zero, no doubt because of her super human goodness. Revealing that would reveal that the evil Emperor Lelouch had been Zero at that time. Such as the fact that Zero killed her when it wold have been just as fast, just as easy, and just as safe for him and the Japanese, to capture her -- in short, he murdered her for no good reason and not much of an evil reason either. Yes, the evil emperor Lelouch's confession probably left out all the details which would have made her worshiped as a goddess of goodness who might have saved the world with a lot less death and destruction.
Making Zero, which the world would think was the same Zero who had killed Euphemia, kill the evil Emperor Lelouch would make everyone think that even though Euphemia was not responsible for the massacre, killing her was the only way to save the Japanese from her. Because if the noble Zero did it, it must have been right and the only thing to do. And so the world would never realize the tremendous potential for good which Zero wasted when he killed Euphemia so senselessly.
This was no doubt one last act of petty spite at Euphemia and Suzaku. They had tried to reform the world peacefully instead of by violent revolution, and so must remain as discredited as Suzaku could be induced to go along with, so that everyone would believe that Zero's way (secretly Lelouch's way) was the only way to change the world. A belief that would not help to make the new world a peaceful one.
And I can't believe that Suzaku accepted that just for a chance to kill Lelouch. I say that if Suzaku really loved Euphemia or cared about what was good for the world, he would have whipped off Zero's costume to reveal the costume of Numero Uno, or Infinity, or Googolplex, or whatever, right before he stabbed Emperor Lelouch to death!