Before he was anything to Iruka, Naruto was just the kid under the bridge. He was what he was, but Iruka couldn't want vengeance on someone ignorant of his crimes.
For five years, he passed the boy-creature, the feeling that bubbled up in his chest slowly warping from loathing to disgust to an odd form of pity, and one day those hollow blue eyes, so full of a familiar pain and loneliness, stared back and he couldn't look away.
So the boy ended up wrapped in his jacket, shivering on his couch, slurping ravenously at a cup of Ramen noodles. And beside him, wondering why he'd invited the devil himself in and knowing it was because he couldn't let a child starve to death, Iruka sat silently.
The boy needed a friend. He needed parents. He needed love. And any of those which Iruka had once possessed were long since gone, and he barely knew how to give them. He could be a father, he could be a friend; he could love the boy despite the monster. But why would he give to this demon what it had taken from him?
Because he needed to be needed.
Because it wasn't the boy's fault.
Because he couldn't let there be another story like his own.
Somehow, there was enough humanity in both of them for tolerance to take root. And Iruka was redeemed, and the boy was saved. Of all the things he lacked, this Kyuubi had at least another name that Iruka could learn to attach fondness to: Naruto.
He could love the boy, could love Naruto, and so he did.
Despite the monster.
And their demons were forgotten, or at least forgiven.