Giraffes aren't necessarily as dangerous as a lion or a snake, and they prefer to run away from danger rather than attack. But if a giraffe is cornered, it can still kill a man with a kick, and they'll fight by whipping their heads around and butting you with the little horns on their heads. They are relatively peaceful, so if that makes them "nice," I guess they are. But if they think you're a threat, you should get away fast.
