Paul was a zealous man, first trying to destroy the spread of Christianity, later serving Christ with an equal amount of zeal. His life changed after a personal encounter withe the living Christ. In 2 Corinthians 5 Paul reminds the Corinthians that when we embrace the truth that Christ died to pay the penalty for our sins, our old life dies. We are to now zealously live not for ourselves, but for our Redeemer. The love of Christ compels us to serve him with all that we are.
For the love of Christ controls us, because we have concluded this: that one has died for all, therefore all have died; and he died for all, that those who live might no longer live for themselves but for him who for their sake died and was raised. 2 Corinthians 5:14-15 (ESV)