Sorry to hear about the loss. Dead on, center chest shots are really never a good idea, especially with a handgun at 100 yards. Very limited kill area. Even though the sight picture looked good, you may have pulled it slightly. If you did, you may have struck a shoulder. I know its hard, but sometimes its better to be patient, and wait for a good broadside, or angling away heart / lung shot, especially with a handgun. That said, if you hunt often enough, sooner or later, you are going to flub a shot. The key to minimizing that possibility is to be critical when you pick your shots.
Larry