I've not read the book though, & the show doesn't really explain his particular raging need to capture Valjean, who after all, only broke parole, and his crime before that wasn't major. This just goes to show how obsessive he is about catching anyone that falls outside the law.
Another reason to love the show though. Even the bad guy isn't really a bad guy, and by the end, you are as sorry about his death as you are about, well, everyone else's.
Well Valjean - broke parole, robbed a priest, robbed a child for 40 sous, abducted a minor (Cosette) etc. He's very much a good outlaw. Javert is so good he's evil - the bad face of being an outstanding policeman.
Javert and Valjean are both based on Vicdocq - the first head of the Surete. One is law with no love, the other love with no law.