You don't know what he needed, he could have hit all three with his last three rounds. That means he may have only got 1 with a 10 round mag 
What you're pointing out is your ignorance on the subject and the fact you know very little about guns.
I bet you're the type of person that sees these police shootings and asks "why cant they just shoot them in the legs?"
just exactly like you don't know he needed a 12 round clip

and just so you're not confused these people were armed and in his house and I'm perfectly fine with this guy shooting to kill
isn't that what you're supposed to do if you point a gun at someone?
I don't see any significant difference between 10 and 12 rounds
I don't see any need for someone to have a 33 round magazine like the Tuscon Shooter or the shooter in Aurora who supposedly had a 100-round drum magazine