What made Shaka Zulu so dangerous?

Shaka Zulu was dangerous to his enemies and to those on
his own side alike.

Shaka was dangerous to those on his
side because of his enormous temper and capacity for brutality.  He was known for this
willingness to execute people for little reason or even for
none.

Shaka is, of course, more famous for being dangerous
to his enemies.  He was dangerous to them mostly because of his ability to innovate. 
Shaka used short swords for killing at close quarters rather than using the thrown
javelins that were traditional.  He used new tactics like his "horns" formation that his
enemies did not understand.  By doing these things, he turned wars into massive
slaughters instead of small skirmishes.