In a sense, Micchy WAS a victim ... like most people in the show... victim of Hellheim, and of the tragedy that happened in Zawame and Earth. While Micchy had certainly issues, it's also obvious that it's that apocalyptic situation that made him lose some of its sanity. That, and the fact he's been forced to betray his best friend for the greater good even before he went completely psycho, his brother who made him feel he was better than the others, and a situation when sacrificing a majority of humanity is seen as a necessary evil to save what can be saved of humanity, and not without reason.
It's not surprising that a young boy like Micchy, who's been confronted with all of that, would start thinking killing someone could be for the greater good, even if he's his former best friend, especially if he's angry with him and sees him as a danger. Of course, the most it went on, the most Micchy lost his sanity, but at the end, the mere fact he was able to realize how much he was mistaken in his action made him redeemable.
In episode 47, Micchy, when he was losing to Jam and Inago, was thinking "were all my actions really pointless and useless", Kouta answered "you're wrong Micchy...you've fought so much after all... " ; Kouta could have said that about all Micchy has done during the show.
Besides, let's be honest : the only reason Kouta was able to save humanity was because he was Sagara's pet, who gave him an incredible amount of power, with the Kachodoki and Kiwami lock seeds. Of course, Kouta had the qualities which were required, (his bravery, and readiness to sacrifice himself), but he didn't have any strategy to save humanity before episode 40 and learning the secret of the forbidden fruit : while Micchy was pretty an asshole during episodes 26-43, one can admit that he was the only one who had a plan to save most of humanity, using his interactions with the main Overlorrds, Roshuo and Redyue: while his plans were flawed, at least, they were there.
Of course, after episode 40, everything changed, and Kouta was aware of the importance of getting the forbidden fruit, and hence his final fight with Kaito, when Micchy became pretty much irrelevant. But when you look at Gaim, you'll notice that Kouta, Kaito and Micchy, all three try to fight to get a better world: Micchy tries to keep things as they were, Kaito tries to create a new world by destroying the former one : but it's Kouta who finally finds the true answer : creating a new world without destroying the old one, even if he has to leave his old world : and as such, as a true selfless one, he's the hero.
Unlike Ryouma or Sid, who were megalomaniacs who wanted power because of greed, and as such, had no room for redemption, Micchy (and in a way Kaito too) had noble intentions but which ended up destructive and misguided.
The surviving heroic Riders (Zack, Jounochi and Oren) were just fighters who protected civilians and Riders, but they didn't have any large scale strategy, so of course, they made less mistakes.
At the end, the reason Zack wanted to forgive Micchy was also because they all tried to fight during the Hellheim crisis, and were the ones to share the hard experiences of the invasion, as fighters. Hence the desire to ne together to keep those memories, and those of the ones who weren't there anymore alive. Besides by showing his regrets and a true and sincere guilt of the bad deeds he's done, instead of being arrogant, Micchy made himself more forgivable by those who had a good heart.
