AFAIK she said she was always a sadist, just didn't know at first.
The entire game would be that one scene where she admits to Edwin that "awoke" her sadism: Some dude in college throwing away his future for her
Honestly we could get that in game like you said it was not a journey it was a door being kicked open
There's nothing personal here (well, besides her getting off on it), she's just making sure that patrons are entertained and customizes her methods so they would fit best to the person in question. Her intention isn't to harm the women that are there but to bring entertainment to patrons. I don't see any malice in her actions. And let me remind you that at any point carnations and whores can just quit (it might be a bit more difficult for some whores who have debts like Kimber but she could still quit and work 2 jobs to pay instead of selling her body).
uhm..... she literally fucks Veronica because of how she acts in the "pre game" and week one, we know for a fact that its personal for her because she later admits that doing stuff like that (enjoying the show) makes all the patrons think of her as just another base whore (admits after Edwins gift) so despite her "losing" ground with the Patrons she still does it, she doesnt have Edwin, or Ian, or a random draw audience member fuck Vero she does it because Vero talked shit at her, you could also agrue that the more recent stuff (felica) is just as focused because not only is Elias being involved but she also has one of the patrons buying a painting, and while the patrons would enjoy cucking Elias buying an emotionally important paitning to felica is 100% for her (kathleen).... unless you think samson or the titty milk guy just happen to really really really like art

imo its more likely that kathleen is doing it because she knows that unlike Elias knowing (this late in the game) the painting stuff could legit hurt Felicia.
I'm not so sure about it as you but it's definitely plausible. She might have injected some of them, although I'm not sure what the point would be if they were just whores who would be doing the same things even without a drug. But judging by Kat's surprise by the result, Edwin was her first guinea pig and she considered it as a reward to him and not something malicious. As far as Kat knew this stuff was harmless (after all she put that on her skin regularly and thus must have absorbed some of it in the process, same went for Sophia who was one of the creators of the drug).
You say that the girls are "property" and that Kathleen owns them? Considering she has injected Edwin without his consent TWICE (if you are a real gamer) during OFF HOURS (not for the patrons), that she has a girl literally trapped in the "basement", that she has given the carnations WINE LACED WITH THE DRUG the idea that she did not do it because "she could" is a wild take.
Also unless im misremembering the laced wine comes before (in story) the injection reward for Edwin he could not have been the "first use"... perhaps the first one of a new batch, or the first male injected, the details arent clear because its been awhile since ive replayed but the idea that somehow they had laced wine first and then somehow got an injection from that seems too far even for Sophie (sorry turret)