Fenrir Greyback is not evil. It should not be said that he is good, but that does not necessarily make him bad. He lays in the in-between, neither light nor dark, but grey. He sides with Voldemort, not because he believes as the Dark Lord believes, but because the "good" and "light" shun and kill his people. Voldemort's promises of fairness to werewolves, is Fenrir's personal cause for following the Dark Lord (even though Fenrir doubts he'll keep them).
Despite what Dumbledore's followers preached to the world, Fenrir did not change children into werewolves for fun or to cause them pain. Did you know the werewolf gene is actually a cure? It can save the half-dead. It kills cancer cells. It savagely fights off infections and disease. It overrides genetic mutations, like the one Remus Lupin suffered from. Fenrir had no grudge against Remus's father; Remus's father brought Remus to Fenrir and begged for Remus to be turned, before his mutation killed him. Fenrir agreed, and turned him. Shortly after, Dumbledore kidnapped Remus; used a memory charm on him, and placed him with a light family to pretend to be his real parents. Fenrir was not able to get close to him until he was already brainwashed, believing all werewolves were evil, mindless beasts.
All Fenrir wanted was a world where werewolves could live without fear. Dumbledore claimed to be a werewolf supporter, so long as you listened to his every order and locked yourself up every full moon. But the wolves inside didn't want that; they wanted freedom to room, to play, and to find their mates in the moonlight. Locking the wolves up full moon after full moon was what was going to cause them to go feral. The inner wolves would rage and turn monstrous, trying to escape into the moonlight.
Fenrir would never hurt a child; only help them, damn the consequences. So on the night night of the full moon, when he smelt young blood, he ran toward it without another thought. When he realized the smell was Lillith Potter, half dead in forest miles away from any other humans, he bit her. He didn't care he was bringing the ministry and Dumbledore down on his head. He didn't care that the Dark Lord would be furious with him. He didn't care he was hunted twice as much as before. He was helping an injured child. When the sun came up, he dropped her off with a young werewolf couple and set up the most powerful wards he was able, and then set off into hiding.
Years past, and many unexpected things happened. He did not expect Lilith Potter to be his mate. He did not expect to become one of the leaders of the creatures: werewolves, goblins, veelas, giants, centaurs, unicorns, hags, vampires, trolls, yetis, ghosts, dwarves, and house elves. He did not expect to guild a third side of the war. He did not expect to plan attacks on both the light and dark sides. Most of all, he did not expect to win.
The final battle had turned into a three way fight at Riddle Manor. Lilith took out Voldemort with a cutting curse to the neck. Fenrir took out Dumbledore with a blasting curse to the stomach. The light and dark sides collapsed without their leaders; it all happened so fast, there had only been 47 deaths: 16 light wizards, 15 dark wizards, and 11 creatures. That was the average amount of dead during a Death Eater raid.
That doesn't mean the after effects of the war and battle were any lessor. The light wizards fought tooth and nail about following and listening to the "dark" and "evil" creatures. As some were punished for attacking a vampire for no reason, they begged Lilith, their "savior" to save them and destroy the beasts! Lilith laughed and sends them to the goblins for punishment. The dark wizards sneer at the "beasts" in charge, but most don't take any actions; they remembered when the creatures worked with the Dark Lord, and how vicious they could be if provoked.
Some days, Fenrir looks around him. He's a leader, in charge of a fair world. No discrimination is allowed: it doesn't matter is you are light, dark, or grey. The economy is booming, new creatures ran new businesses with new products. He's a husband; he and Lilith married after the final battle. She was a wonderful co-leader, and happily took over most of the paperwork (Fenrir has a bad habit of getting sick of filling it out and setting it on fire) and often takes care of the light side problems (they listen to hear better they do him, because he is too dark for them). They've decided to try for a baby in the next few months.
Fenrir got pretty far in life for saving the life of a child. It wasn't was he, or anyone else in the wizarding world expected, but he wouldn't change it for the world.