"Hit him harder, the bastard," yelled Ali Khosravi, whose 10-year-old son was last year kidnapped and killed by 22-year-old Mohammad Bijeh.
But Wednesday was the killer's turn to die.
"I would like to strangle him and burn him myself," Khosravi said of the man convicted of killing 21 people - mostly little boys.
Before his execution south of Tehran, Bijeh, his hands tied to a post, was lashed 100 times.
The huge crowd, kept back by barbed wire and security forces, chanted "Harder, harder!".
Bijeh, branded "the vampire of the desert" in the Iranian press, was arrested with an accomplice in September after more than a year of luring children into the desert before beating them with stones, sexually abusing them, then burying their bodies.
Bijeh also reportedly ate a victim's leg - just to see what it tasted like.
Just before the hanging, the 17-year-old brother of one of his victims broke through the security and planted a knife in Bijeh's back.
Then the mother of another victim was invited to place the noose around his neck.
He was then hauled into the air by a crane.
"Dance and think of what you did to our kids," yelled a father, as Bijeh was throttled to death.