IMHO....I think Terri was the one having an affair with the landscaper so that she could manipulate him into killing Kaine....
Also, Kaine disputes Desiree's version of events of how their marriage ended. Obviously, both of them see things differently. But I don't think it's entirely fair to accept Desiree's version without giving Kaine's version any merit. The truth is always somewhere in the middle anyway, and it's obvious that they were not communicating well by that point.
I believe that Terri disliked Kyron because he was Desiree's child with Kaine. Kyron was also a focus of Kaine's attention and love.....and money, as his only child for several years. Terri wanted everything for herself. I think she got pregnant to ensure a continued income for herself, and to further bind Kaine to her, and to take something away from Desiree as the mother of Kaine's child. Now Terri also had a baby by Kaine, so Desiree was not that special anymore. Desiree is a beautiful woman, and I think Terri was jealous of her in so many ways.
I believe Terri also saw Kyron as an intruder into her plans, and as competition. I also believe her anger at Kaine stemmed from battles with him about Kyron, and about her drinking. I think she planned to do away with Kaine, and get his life insurance money, his property, and SSI for Kitty. Kyron would go to Desiree, and Terri would be all set until she found a new man. No divorce to cut her out of the money....
When that didn't work, Terri planned to get rid of Kyron because he was an obstacle to her plan of being Kaine's one and only, along with Kitty, who would then be Kaine's only child. Desiree would be out of the picture, and Terri would be queen of the castle.
I don't believe Terri has the capability to feel love, or to understand it. But she does appreciate money and status, and the comfort of having a man to provide financial security. That's what her marriages were all about....she was always financially motivated, not motivated by real love.