Walter Mabry: This is the key to every computer system on the planet. I want *you* to steal it for me.
Arthur Tressler: Hell is going to seem like a day at the spa by the time we're through with these Horsemen, wouldn't you say?
Walter Mabry: You had me at "Hell."
J. Daniel Atlas: We are going out with a show people will never forget.
Answer: Walter thought the chip was real because he placed it under a scanner and the scanner confirmed that the chip was real. However, given the fact that later on it was revealed that the whole plane sequence was a set up, it is probable that the device used to scan the chip was actually a fake that was used to make Walter believe the chip was real, most likely throw him off guard.
Casual Person