So basically what you're saying is because he missed the kill on the first shot he's now the bad guy and the "injured and gasping" criminal is now vindicated.
Where do you stand? Do nothing? Only allowed one shot? Shouldn't use a gun at all?
You Sir are messed up. Breaking in does not give the homeowner an open license to kill you, breaking in the homeowner an open license to use REASONABLE FORCE to defend themselves. Reasonable Force means that they are allowed to SHOOT you, or SHOOT TO KILL you, up to the point where they are no longer in a position of having to defend themselves anymore.
If they are still alive, you don't get a "second shot" to kill them (You are allowed to defend yourself, and executions does not fit under defence), you call an ambulance to attempt to save them (even if you think it will be futile or they will die a slow painful death while waiting) and the criminal justice system takes over
Basically when you are shooting to kill them, the aim is to kill them so that they can't harm you, the aim is not to kill them as a way of giving them your own justice.