I think she's so very toxic towards Kate because Kate is first in the pecking order. She wants to be front and centre of everything g, she can't do that in the UK because she's not going to be queen (thank god!) and she's way down the list in importance.
Floofpaws - and I think the raging jealousy of Catherine started even before she met Harry. She's known to have written disparagingly (but clearly enviously) on The Tig of W and C's wedding. It was all so predictable: Wannabe Hollywood actress (she was only ever a cable actress), brought up in California, meets English Rose with cut-glass voice and a reserved manner. It was a disaster waiting to happen, especially since Harry was once so close to Catherine and clearly very fond of her. And from Meghan's point of view at that time* there was nothing for her in going along with the status quo. Put those factors together and you have the material for a fair-sized nuclear bomb.
There was also the time when they first appeared together in the royal box at Wimbledon and Meghan suggested that, instead of following protoco (M following Catherine) that they should both hold hands as they entered the box. Catherine declined (and blimey, so would I - what a ridiculous suggestion) on the basis of protocol - she'd been instructed what to do and she wasn't going to change that for Meghan's bonkers ides. But of course Meghan took mega-offence. It's all so utterly childish, and borderline farcical really.