So supposedly Zanny was a ten who came from a lot of money. Didn't KC also say she graduated from college? She supposedly had a great relationship with her family and roommates she was very close with. If that was the case this girl had a huge future ahead of her and her life together. So instead of taking advantage of this, she becomes a nanny for free, kidnaps one of the 2 children she watches for no apparent reason, and then kills her, leaving the body on land where her DNA or fingerprint evidence could be left on it.
Now take KC. A girl who didn't graduate high school, has to lie about having a job, needs to steal money from her parents, grandparents, and child, steals gas cause she cant even afford it, and has friends who constantly ditch her due to her lying and stealing. Her own mother referred to her as a sociopath, she wanted to be committed at one point, and she was constantly fighting with her parents over her daughter.
Hmm comparing these two (based on the description KC gave of Zanny) it seems like KC had her life way less together, nothing going for her, and nothing to lose. KC would have had a motive and the psychological problems to back up a crime like this. Why would "Zanny" go through all this just to teach her a lesson? So yeah, blame it on the perfect 10, rich, responsible "nanny."