A lot of headcanons, a lot of fan theory, a lot of unexplained stuff. I wrote as if the kwami were a bit more like gods who weren't necessarily tightly bound to their Miraculous and could influence the location of their Miraculous somehow instead of being entirely dependent upon being delivered or stolen by someone. I just felt like this could be a good take on Nooroo's character. Sorry if it doesn't make much sense. The ending's kind of abrupt and not quite what I wanted, but I just can't get it right, so...
Nooroo, contrary to popular belief, wasn't stolen nor was he forced or coerced into anything. He willingly allowed his powers to be used with impure purposes.
The moth kwami was the one who was always closest to the humans, wanted to protect and help them. His powers allowed him to grant someone of particularly strong spirit a strength unique to them, even without a wielder. The powers that he granted by himself weren't as strong as they might've been with a wielder, but Nooroo always took pride in the humans and how they persevered over the centuries, even without much guidance from the kwami. Then why? Why would he willingly let his powers-intended for aiding and strengthening humans-be used to make others suffer?
That's because Nooroo was a kwami easily swayed by pity and sympathy. Suffering happened all over the world. It was a fact that Nooroo begrudgingly accepted, as much as it pained him to do so. And as much as he wanted to deny it, his powers had as much destructive power as the black cat's if used to aid someone with a particularly strong hatred in their soul.
Of course, any of the Miraculouses could be dangerous when in the wrong hands. But the other kwami would never let themselves be abused so easily. Nooroo, on the other hand, felt sorry for those who faced such terrible circumstances that they became so filled with hatred. Breaking the common rule among kwami, Nooroo became too involved with human affairs.
Among all the humans within his reach, Nooroo chose the one with the most hatred, the most sadness, the most grief, the most suffering and offered his Miraculous to them, informing them about the wish-granting power of the Black Cat and Ladybug Miraculouses in order to draw out a Ladybug champion.
And with his power, Nooroo drained a tiny bit of his wielder's hatred into his butterflies, into the akuma to infect a victim. Once Ladybug purified the akuma, the victim would be safe and a bit of his user's hatred would disappear. Like this, Nooroo was able to ease his partner's suffering.
He knew it was wrong. He knew that it just caused more suffering. Sometimes he wasn't able to absolve his partner's hatred before they died. Sometimes he questioned why he was doing this at all. But whenever he came across another human suffering with so much intensity that it layered the air with the sensation of death, Nooroo couldn't help himself.
And that's how he found himself in the present day, watching over a man grieving over the loss of his wife. In his grief, he effectively pushed away his own son and poisoned his former partner, Duusu, with his negativity. For Duusu's sake, for the son's sake, and above all, for the man's sake, Nooroo watched over him. Granted him the ability to wreak havoc and fright over Paris in order to grant his salvation.
Well, Nooroo did think it was a bit cruel to make his son the owner of the Black Cat Miraculous. It would either make things easier in the end or would create a point of no return for Gabriel. Nooroo hoped strongly for the former. But even if there was a happy end for Gabriel, Nooro knew that there were always others who were suffering. Ones that could find temporary comfort in his power and eventual salvation through Ladybug.