I didn't think it could either until about a year ago not far from where I live.... this happened.
http://www.wsbtv.com/news/14637705/detail.html
Six year old shoots 5 y/o sister, kills her.
A pellet gun, as was stated in the story, will toss a BB at about 3X the speed of a BB gun.
Still, this is tragic. Adults required, and not just someone over 18 or 21, but real grown-ups.
Pellet guns are significantly different from a BB gun.
Some pellet guns shoot at 1400 feet per second. The average BB gun shoots and 300/400 feet per second.
Only a shot into the Eyesocket with the Eye already out could kill someone.
It was not a BB gun. There is a big difference. Some pellet guns approach low powered cartridge arms in energy of the projectile.
Boy, 6, Kills Sister With Pellet Gun