Der Kindestod was a child-killing demon who seemed to be influenced (in terms of appearance - dress sense in particular) by horror icon Freddy Krueger.
It killed children by literally sucking the life out of them. Afterwards, it looked like they had died of fever. Therefore it preyed on children who were already ill, leaving little evidence to suspect their deaths were unnatural.
Its killing method made it a truly terrifying being. Rupert Giles describes it best (when he finally gets the words out, at least):
Giles: "Uh, the, um, the Kindestod gorges by sitting atop his prey, pinning it down, uh, helplessly. Then he slowly draws out the life. I-it must be, uh, h-horrifying for the victim."
It was also a very capable fighter, as it gave Buffy a run for her money, showing combat prowess and strength far superior to that of an adult human.
The stories and myth of Der Kindestod alone are enough to fill children with fear. He would be a welcome addition to the Sinestro Corps.