Yes Kendra's goal is to destroy David and Anna's perfect marriage it's even if the ex is planning all this I have some doubts about her at the beginning of the story she throwing away those medicines in the bathroom and planning her revenge you can tell that love that she has for David is still there
Even if it is a sick love of a crazy woman who is hurt hurt and wants to make David suffer but deep down Kendra what back and she will do everything to get Anna out of her way like blackmailing Anna like you said slowly corrupting making him sleep with other men and with that she will be keeping several proofs of the cheating wife
And then just show everything to David and get what's hers back but she's going to make David suffer a lot and then win him back and in that she shows that he made a mistake in abandoning her