Originally Posted by Toasty
The reason he took Clementine and not anything else, is because he wanted to take the one thing they care about the most. Since they took something from him which was life and death, they wanted to take something from them that would make them come looking for who did it, and then he could kill them for what they did to him and his family.
I dunno, I think once he found Clem on the radio, it was less about getting revenge on the group and more about getting to Clem and punishing Lee for being a bad father (and therefore, fixing his own failures and proving himself to be a better one). He even said, in my play-through, that he was going to leave Lee alone until he heard all the things Clem told him about.