No, what I really think is: When the owner of a deadly weapon is physically contending for possession of said deadly weapon with a thief, it is reasonable for the owner of the deadly weapon to be in fear for his life and thus able to act in self defense.
I think this holds true across a wide range of scenarios, including the ones we were discussing. On the other hand, you seem to be trying to generalize the deadly weapon as some arbitrary item, not a deadly weapon. Obviously if you substituted "harmless ice cream cone" for "deadly weapon", the circumstances will be different.
hmm, maybe the part that you are missing is intent. If the perp is coming at you or not even fleeing (armed or not) and you feel threatened, by all means blow them away.
The problem of intent comes when the perp is not approaching or threatening you, then you lose the self defense justification. You don't lose all your rights, but you had better be very careful about the use of deadly force. If you are approaching the perp and/or threatening the perp (even if you can see that he has your stuff) you have become the aggressor. The perp at this point in time could be justified in killing you because you are threatening him and he has a right to defend himself.
The only exception that I know of is the Castle Doctrine. Generally if the perp is in your house it is automatically assumed that you are being threatened and you are free to use deadly force to your hearts content.
Outside of your house though, if the perp doesn't initiate the confrontation and you do, your use of deadly force will be considered murder. Think about this carefully.