Killing a friend - Detective Riddle

Kushal is accused of killing his friend Riya. His lawyer in his final statement declares that Riya, in fact hasn? been killed and will enter the courtroom in a minute. Everybody in the room, including the judge and the jury, turns and looks at the door, and the lawyer with the defendant looks at them. Several minutes pass and the lawyer says, 'It proves that you are not so sure that my client has killed Riya! Otherwise you wouldn't believe me!

Almost immediately after that, the jury comes up with a guilty verdict. Why does it happen?

Kushal, who was accused wasn't looking at the door. He was sure that Riya wouldn't enter as he had killed her. Kushal, who was accused wasn't looking at the door. He was sure that Riya wouldn't enter as he had killed her.