Even if he was armed, would presenting a firearm of your own just increase the chances he would actually use his firearm? He is there to steal, not to kill. Remember that burglars generally do not like killing. Even if a burglar does have the stones for it (and this is by no means common, as killing is far different than stealing), they would prefer not too because they run the risk of spending the rest of their lives in jail. So I think that, if the burglar is armed, you are only increasing the chances of your own death by confronting him with a weapon of your own. If you are unarmed and he waves a gun in your face, you can back off and he will more than likely flee. In any case, the instant he realizes you are there then his own behaviour will be dictated by your own. Threaten to kill him, and he will fight for his life.
Creeping up and opening fire on a trespasser, without warning or being aware of whether or not he is armed, can't be right in my eyes.