Board Thread:Questions and Answers/@comment-26955371-20150920164120/@comment-5780397-20150920181639

Aizen had always known Gin was going to betray him, but he was not expecting Gin's attack. Of course, Ichigo had difficulties against him, but it wasn't impossible for him to win: he had defeated opponents thought to be much stronger than him, and Gin knew it. Although again, I believe he lied either out of habitude or not to give Aizen any chance to find out he was hiding something.

When Gin stabbed Aizen and told him his sword was neither as fast nor as long as he claimed, I reckon that was also a way for Kubo to inform the reader. Mach 500 is already fast enough to kill a Shinigami captain before they can react (Ichigo clearly stated that it wouldn't have been possible for him to dodge or defend), he wouldn't have needed to exaggerate its speed if that was the case. Furthermore, it is only natural that Ichigo would be unable to see Gin's sword, if it disintegrates right before it attacks. However, these are merely secondary conjectures, my main point is still that Gin lied to preserve his secret, and that he had no reason to help Ichigo defeat him, since he was convinced he could kill Aizen himself.