King Acrisius fears Perseus, his grandson, because Acrisuis was told by the gods that he will never have a son, and that his daughter's son (who turned out to be Perseus) will kill him. Having heard this, he kept his daughter isolated, but Zeus went ahead and got her pregnant, persumably to mess with Acrisius or maybe because she was very beautiful. Due to the prophecy, and because Perseus is a demigod, Acrisius is quite afraid of Perseus.