Excerpts from wikipedia on "narcissistic rage"
"Narcissistic rage is a reaction to narcissistic injury (when the narcissist feels degraded by another person). When the narcissist's grandiose sense of self worth is perceivably being attacked by another person, the narcissist's natural reaction is to rage and pull-down the self worth of others (to make the narcissist feel superior to others)."
I think the fight on the night of the 15th was a narcissistic injury that caused the "house of cards to come tumbling down" There's no telling what CA said to KC that night but I'm sure it made KC feel degraded in the worst way. CA had always covered for KC but not any more; not after SP confronted CA about KC's lies and theft. CA was probably brutally honest in her confrontation with KC and this change shocked her. KC was no longer on a pedestal in CA eyes. It brought on this "narcissistic rage".
"According to Kohut[1], rages are a result of the shame at being faced with failure. Narcissistic rage is the uncontrollable and unexpected anger that results from a narcissistic injury. Narcissistic injury is a threat to a narcissists self-esteem or worth. Rage comes in many forms, but all pertain to the same important thing, revenge."
Again, the confrontation with CA may have made KC face her failure and she was shamed to no end. CA probably threw it in her face how she was the only one mothering Caylee and what a lousy job of being a parent KC was doing. This, along with all the other things CA could have attacked her with, brought on this rage. KC sought revenge against her attacker, CA and the one who knocked her off her pedestal, Caylee.
"They can become enraged to the point of being homicidal especially if he/she has the need to seek revenge."
What better way to take revenge on CA than to take the life of Caylee? KC could no longer be attacked on her mothering skills and CA could no longer take credit for all she did for Caylee. KC knew how much it would hurt CA to lose Caylee. In a display of power and superiority over CA, KC took what meant more to her mother than anything in the world. "I gave you Caylee. You put her above me and you used her against me."
"What is given can be taken away"
CA probably knows nothing of narcissistic rage/injury but she does know that the fight they had that night was the proverbial "straw that broke the camel's back". This is what I think she feels guilty of. She feels as if she should have kept quiet and Caylee would still be alive. She thinks that if she had left KC on her perceived pedestal this wouldn't have happened.
MOO
http://en.wikipedia.org/wiki/Narcissistic_rage