I think she wanted to go out and be with her boyfriend, but she was furious with her mother, so she couldn't ask her mom to babysit, so she knocked Caylee out. She put her in the trunk to "sleep", put tape over her mouth in case she woke up and started making noise. But I think she left the rag in the trunk. The heat, the humidity, the confined space, that soaked rag... too much and it took Caylee's life. When Casey realized what happened, she didn't know what to do, so she drove around with her in the trunk for a few days. She later then left her in that black bag by the elementary school.
She did want to punish her mother. She did feel like Caylee was the darling of the family. She was jealous of her own daughter. I also think that she wanted Caylee out of her life so that she could try to pursue a future with Tony. He had made mention that 1. he might move away and 2. he wanted all sons because of what he had witnessed with the girls in his family. Remember Susan Smith? She thought that she could eliminate her children, get loads of sympathy, and still get her guy. I wonder how much of that factors into Casey's situation.
Right on the money with inclusion of "I must be a spiteful ". it took 45 seconds to drive to the dump site with lights off and another minute or two in and out to dump the bag in the woods she was familiar with. She could drive off in the dark, without being seen, in a total of four min min or less.