Antagonist
Character Role Analysis
The Death Penalty
Dead Man Walking is short on actual human-shaped antagonists. Pat and Robert are violent murderers—but they're not fighting against Prejean. In fact, she's trying to save their lives. There are lots of people involved in executions, from the governor Edwin Edwards on down to the guards who pull the switch, but Prejean doesn't portray any of them as evil. The victims' families are opposed to what Prejean is doing, but they're not antagonists. The Harveys are her friends; she wants to help them, too.
So what we're left with as antagonist isn't a character, but a thing: the death penalty. Capital punishment is the enemy: that's the thing Prejean wants to kick over and step on and banish into outer darkness. In Prejean's world, people aren't evil—but killing is. So killing is what she's fighting against. You could have titled this Nun Wrestles Death Penalty. Then again, maybe not.