Hmm She looks quite clean now, seems the mister cleaned her up, and is using her as a foil for his JUSTICE plan by reversing the initial situation now with the shit kos not allowed inside.
It doesn't take much that this was all the anon's intention to convert them into shitheads. Why else would he just leave the "good" mother outside so it could see how good the "bad" koyukuri's are treated and how they turned into real shitheads while at the same time increasingly insult there mother.
I don't think his intention was to "convert them" into shitheads, maybe he wanted to check if their claims of having being pets and being good pets was true, and if that wasn't the case he would have got himself a brand new abyuse fodder.
The Mister only puts them in his house and the little shits start doing their claims of greatness, slavery, etc.
Also remember the mother asked him to take care of her kids, maybe she followed them and checked if they were fine, but then the little shit showed their true colors, the mister saw this and decided to try a reverse scenario, that, while good to spite on the kids, didn't turn out much better because the mother was a potential shithead as well.
Long made short This is what people would call "take them home and test their shitheadness before proceeding to officially making them pet if they aren't or abyuse them to oblivion if they are."
I agree on that. Like I said about the shithead elder Marisa in the Shell Marisa story, a shithead could look nice enough at first, then show its true colors once it's comforted. And it looks like this mother Reimu really was a badged pet once because the kos are saying she forced them to study. Just serves to prove not all pets are nice. The reverse, as in not all stray/wild yukkuris are shitheads stands too, of course.
1) The mother asked to adopt them all, not only the children. 2) The anon only took the children that showed signs of shithead beheaver. 3) there is something called reponsibility, anyone who knows something about yukkuri's knows that you can't these children do as they wish, the anon didn't do anything and even more he lets the mother see how there little ones degenerated further while she could do nothing about it. so either this anon's is dumb or he did it on purpose. And the last page hardly suggest the latter.
Or the Anon did the "shithead" test, specially considering that the mother was arguing with them at the beginning about "greeting properly".
Yukkuris easily show their true colors at the first moments of comfort, so the mister did a flawless test, separate them give them confort and see how they react, if they reacted badly, he would switch the situation and see how it developed, the Mother also showed her true colors as well when she got comfort and a chance of revenge.
So the Mister could conclude this yukkuris were unworthy shitheads and proceeds to bully them.
If the anon wanted just to bully them, he could had done it since the beginning, no moral restrainment or need to see the Worth, should restrain him of abusing them on the spot, shithead or not.
And since this anon seems, based by closely similar scenario, the kimeemaru and such, to be the anon of Compost Yukkuri, we can think he likes to "test" the shitheadness of the yukkuris to some degree, and if they show blatlant shitheadness he proceeds to abyuse them, but always making sure to test the whole set, not only the kids, or only the mother, but all of them.
Patience is a very important factor in testing, specially in a swap and change test like this one. Reason why he went "well can't take it anymore, abyuse time!"
Either way all this is theory based on various aspects, from the fact that he isn't bullying them at spot, something that someone not caring if they are shitheads or not would do, trying with each element of the family before reaching veredict. "Corrupting" is pretty much pointless, Abusers don't need reasons to abuse them, they just do it, as much, mindfuck them a bit, but nothing like some sort of "making the corrupt", and people who hate shitheads only need to test them to get to know the yukkuri's worth or pay attention to their mannerismn. There is also the whole deal of this anon being the same one of the compost yukkuris.
So it's hard to believe the whole "corrupting them" specially because how yukkuris work and the initial conflict between their whines and her mother at the very beginning of the story, way before being "adopted".
I think it would be extremely funny to corrupt a good yukkuri. In a kiriraitaa story Mister almost forced a koyukkuri losing her will to live, so I guess that it would be quite... easy. There was that other great story about 2 Marisas only 1 hat...