Do you think t is better to meet an injustice with revenge or to turn the other cheek? Do you think Cassie should have taken revenge on Lillian Jean?

I think one's personality comes into this. Cassie is a firecracker. She is intelligent and feisty. She refuses to take abuse lying down, even if the odds are stacked against her. She figures out a pretty shrewd plan against Lillian and executes it. Given Cassie's intelligence and personality it is great she stood up for herself.