This is why you keep a sanitized throw-away revolver or knife around the house, so you can put it in the dead perp's hand before the cops get there.
Technically though, you are allowed to defend you life and/or your
property - they don't have to just threaten your life before you can respond with possibly lethal force. I can tell you this, I am going to meet you with a firearm if you break into my house, regardless of whether you are there to "injure" me or not, whether you're armed or not. You've committed trespassing and breaking & entering up to that point, I am not going to give you the opportunity to add aggravated assault, assault with a deadly weapon and/or murder to your list of charges before you find a laser dot in the center of mass...you'd have a few seconds to decide: instant death penalty or taking your chances with the trespassing and B&E charges once the cops get there...since ADT called them for me the instant you broke in.