If you mean why (assuming that my theory that TH planned and took Kyron is correct) TH did what I think she did when she did it, then there could be a number of possibilities. I'm not sure what prompts anyone to decide "this is it. I can't take anymore and I'm doing this now.", a la scott peterson, Susan Smith, casey anthony, Diane Downs, etc. But in all of these cases, I believe that the person or persons they disappeared or killed became the source or symbol of their anguish, in their minds, over time, and they began planning how to get rid of the "problem". I think they either look for the best moment
possible to do what they want to do, or something specific happens that makes them determined to create that moment, instead of waiting for the "right" time. For example, I believe casey anthony decided in March 2008 to get rid of Caylee. She began thinking and planning. Then there was a terrible fight between her and her mother. That's when the planning sort of ended and the plan became a sudden reality. Just my theory.
I don't know TH or any of the people involved in this case and unlike some of the others mentioned above, I don't feel I have anywhere near the info I need to create a halfway credible psych profile. So my guesses are based on little things here and there, comments by people who know TH, her own e-mails, FB, etc. It's not enough for me to feel confident as to exactly what makes her tick. But my sense is that she could be the type who needs to feel adored and sexy by the person she's with. Hence, the several marriages - when the excitement of the new relationship ends, she gets out. (I'm just supposing here.) Perhaps the reality of marriage and family with a guy who seems to me not to be too emotive, kind of technical and perhaps wooden, began to press down on the fantasy of creating a family with a guy who left a more attractive woman (JMO), for TH, who maybe never felt attractive enough. "See? I am beautiful. I'm better than this woman who thinks she's so pretty. Her husband left her for me even though she was pregnant!", or whatever.
I have a friend who's like this. It's sad. She flits from man to man, marriage to marriage, very insecure about her looks, constantly getting bigger *advertiser censored*, working out, redying her hair, having extended photo shoots of herself for no reason. And it seems her goal is to prove to herself that the guy she has her eye on, really, really wants her. The sex will be fantastic at first. She'll know this is the one. Everything she does will be to get this guy. And when she does, when he proves he wants her and wants to be committed, within a couple of months, she's done. It stems from her being a mousy but adorable little girl whose parents were cold and withheld love. When she became a teen and realized she could use her sexuality to get attention, boy she was off!
In any event, TH may be someone like this. Now she finds herself in a relationship in which the fantasy has faded and the reality is not so good. Maybe she begins to blame Kaine for the fading of the fantasy. Maybe she begins to also question her own worth and attractiveness as the reason reality is not what she expected. She may not understand that the excitement and passion of any new relationship cannot be sustained for long, especially when bills and problems, babies, barf, illness, and fights, sleep deprivation, work and the guy farting in the bed next to her, become the day to day. But she doesn't understand that this is normal. And due to deep-seated insecurities that she cannot face, she blames others. And due to possible other, more abnormal problems, like PPD, or a personality disorder, or whatever, that blame takes on a dangerous focus: "It's Kyron. He is the reason our life is not so good. He's always been in the way. If it was only me and Baby K and Kaine, things would be better. Kaine doesn't love me and the baby as much as he does Kyron. He needs to go. He's the problem. That will show Kaine too. I'll get him for not giving me what I deserve. Then, he'll learn. And DY too. She thinks she's all that. I know she does. She thinks she's a better mom than me. She thinks she's prettier than me. She probably thinks she can get Kaine back too. Well, I'll show her. She'll never get Kaine back and now she won't get Kyron back either. Then they'll see."
I believe that the thinking or criminals is convoluted and their reasoning complex. It doesn't make sense because it is abnormal. For example, in casey anthony's case, I think she saw Caylee as a burden who prevented her from being free to party, and also as a source of constant criticism and anger by her mother: "You aren't a good mom. You need to get a job and stay home and take care of your own baby." At the same time, she saw Caylee as usurping her place/role as the adored and spoiled princess of the family. And, she was horribly jealous and angry at her mom for trying to take her place as Caylee's mother. So, killing Caylee was a way to get rid of the burden, get back her place as princess, and punish her mother for criticizing her and for trying to be the mother to Caylee that casey was supposed to be. It's a mess.
Similar thinking may have taken place here. I do not believe this was precipitated by any affair on Kaine's part or anything like that. It was probably a combination of things for Terri that led to a growing obsession that Kyron was both the problem and the means for getting back at those TH feels wronged her or posed a threat to her. I believe this was planned but I don't know why, on that day. It could have been opportunity, it could have been a comment, anything, really. It's hard to understand a mind as disordered as that of a person who could harm a child. Who knows.
These are just theories I'm putting out there based on the scant info I have which may or may not be reality. It will be interesting to find out more. The reasoning of criminals and their psychology is something that I'm always trying to figure out. I guess as a way to perhaps protect myself and my loved ones from similar people that may come into our lives. But there is so little actual info in this case that I can only imagine.