The "Angel of Death" scene occurs in chapter 6 after Pap gets drunk. Pap often drinks enough whiskey to make him "blind drunk," and Huck bides his time, planning to make an escape as soon as Pap passes out. Unfortunately, once Pap passes out, he doesn't rest well, thrashing and tossing in his sleep. Huck tires of waiting for his escape opportunity and falls asleep himself.
Huck awakes to chaos. Pap is up again, screaming and looking "wild." He believes snakes are crawling all over him, but Huck doesn't see any such evidence. Pap begins grabbing at his neck, begging Huck to pull off the snake which he believes is biting him. Although there are no snakes, Pap works himself into such a frenzy that he finally falls to the ground, exhausted.
For a moment, it looks as if things might calm down. Then Pap begins whispering that "they" are coming for him. At this point, he believes "the dead" are coming. Terrified, Pap rolls himself up in a blanket to hide. When he sees Huck, he calls him the Angel of Death and begins chasing him, intent on killing him so that he can't "come for him no more." At one point, he catches Huck, who manages to slip out of his jacket and therefore avoid being stabbed.
Finally, Pap is once again exhausted and falls to the floor to rest, telling Huck, whom he believes is the Angel of Death, that after he sleeps for a while, they "will see who is who."