She kidnapped the liars because she viewed them as her dolls. To Charlotte, the Liars belonged to her and her only. She wanted to play with them forever, but knew she needed a secure place to hold them. Hence, the underground dollhouse bunker. The reason she was so obsessed with the Liars is complicated. According to Charlotte, when Mona was in Radley, she told Charlotte that the Liars were happy that Alison was gone. Charlotte is a mentally unstable woman, and she was enraged that the Liars were happy that Charlotte's sister died. She also became obsessed and addicted to the game and being in charge, and never wanted to let that addiction and sense of control go. So, she kidnapped the Liars and made it look as though they escaped prison so no one would look for them. This obviously didn't work.
I hope this answered your question!
