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.