Yep, I completely agree. However...I think Casey had a love-hate relationship with Cindy. I believe the love component of that resulted in a jealousy that would not allow her to give Cindy anything she could love more than her. And I think that oveer Caylee's short life there were times when Casey was jealous of the attention Caylee got--much of which was probably just a natural attention that always goes to a young child, i.e., they can't make their own meals or wash their own clothes or vacuum the rug, or earn money to buy their own clothes, toys, etc. I think Casey is just odd enough that she would resent any number of things her parents did for Caylee that they used to do for Casey. She saw it as her mom loving Caylee more. For me that's easy to consider because of the fact that Casey was so jealous of Jesse loving Caylee more that she actually left Jesse, taking Caylee out of his life.
The hate part of the relationship reared its ugly head from time to time when Cindy said or did something that pizzed Casey off--refusing to babysit might be an example, yelling at her for stealing from her grandparents would probably be another.
If Casey was mired in the hate aspect on that fateful day, Caylee's death may have been not just to rid herself of the "little snothead" but to stick it to Cindy in the process. Certainly not the only reason for the baby's demise, but one that I see as plausible.
JMO, all of it.
ETA: Totally different situation but I think Josh Powell killed his boys to hurt the Coxes. He knew they loved the boys and wanted permanent custody of them. JP was planning on leaving (suicide) but with his evil mindset he also wanted to make a statement. Casey was not suicidal at all, but she wanted to both rid herself of her child and to make a statement. MOO