Damaged pride

Blinded with rage, Leon pulled the slider of the handgun to recoil the bullet. He was at the brink to fire the bullet through Aaron's head.

There was still no fear in Aaron. Before Leon could recoil the bullet, Roach could have shot Leon's hand or shot him to death. 

That wasn't the intention of Aaron. If Aaron really wanted to do it, he could have done it in the past two years right after knowing Leon Harmon had the hand in his Cupcake's state three years back. He had to control as his Cupcake had a grand plan to avenge Leon Harmon.

Taking Leon's life wasn't the retribution his Cupcake wanted for Leon. It was probably the easiest way against the pain Leon had inflicted in his Cupcake and many other women who had gone through it.

If Leon dies without giving out the moral to the cruel world, there was no use in avenging him. If anybody even thinks of being like Leon, they should tremble knowing how Leon ended up suffering and regretting all his life.