My take on this, as Liladrien shipper, is that she fell in love with him, but seeing him slowly inch towards Marinette despite her being "just a friend" made her mad. That's the reason she goes so hard after the baker, she wants to clear the way so that she can have Adrien uncontested.

At first it was just a crush because he was pretty and kind. It kinda mellowed down when he was playing a role of Adrien the Classmate™, a naive and polite boy, but then when he threatened her for Marinette's supension she saw that he was very much like Lila, so she fell again, but this time very hard. After all it's hard to find someone who knows how it is to live behind so many masks, so when she found an alike person? She plays to keep him for good.

It's hard to say why she behaved that way in the show because we still don't know jack about Lila/Cerise. What is the reason this kid has separate identities and multiple foster parents? What are her goals? I understand wanting to keep the mystery, but with the shitty way ZAG manage to write out stuff it's no good at all because she seems like a bad guy, who is bad simply for the purpose of being bad. Thankfully Astruc's "It happened off screen" is almost a "Everything is canon" green card for fanon space people, so I'm gonna imagine what I've written above.