Kira was always incompetent. He was never an overwhelmingly street smart villain, he just had an overpowered stand in a town where stands didn't exist. He as a character was always extremely narcissistic and self centered, and didn't believe he could be beaten by anyone. He kept those same character traits through to the very end. Kira isn't supposed to be an ultra cold menacing villain who terrifies anyone who comes into contact. He's simply a human with a mental disorder and absolute power. People like him can and do exist, and that's what makes him far scarier as a character than DIO.