Amateurs. One pound of C-4 creates a shock wave that kills several meters away from the detonation. Death is from the instantaneous brain damage, augmented somewhat by being enclosed within a heavy structure.
Explosives don’t “atomize” people unless they are in direct contact with the detonation. The dead shooter is wearing a heavy ballistic vest, which would have mitigated some of obvious physical damage.
To give you perspective, we used to use one pound of C-4 to destroy heavy bunkers in Vietnam, usually sending the log roofs far into the air. The dead occupants looked a lot like the shooter afterward.
Thanks for your insight. Seems quite plausible.