The Lone Wanderer may help him escape, or get him killed with a high enough speech skill or Charisma (negative Karma). If you kill the preacher or he survives the explosion, he will simply crouch down and stay in the alley.
"He's... he's been up there for days! He's got all of those mines wired to a trigger. He won't let anyone in or out of the alley. If I try to leave, he'll set them off. But I can't stay here any longer!"