There's no hard and fast line to be drawn for 'fearing for your life' so yeah, an unarmed attacker could still cause you to fear for your life and allow for use of a deadly weapon. Put it this way, I'm absolutely not going to get into a fistfight with someone who has entered into my property illegally simply because they don't appear to be armed. Now I'm not saying I'll just whip out a pistol and start shooting but that all depends on what that person is doing, and more importantly does, when my firearm is produced. If they comply and go prone or turn and haul maggie out the way they came, then I'm not going to finish depressing the trigger. If then don't stop being a threat real, real fast, then it's going to get bad for them. It's not really my problem. I didn't tell them to break in.
On your second point, I'm not actually sure how much that matters. Once they dead guy produced a deadly weapon, all bets are off. Invited. Uninvited. Whatever. Assuming the story is true and he comes in and initiates an attack with a deadly weapon, he's opened the 'kill or be killed' mentality. The fact that he lost is, once again, too bad. Even if you are an invited guest on someone else's property, instigating a deadly conflict still provides a justifiable defense.
Now all that said, if that's not how it went down, then all bets are off. If we find out the guy that lived actually started the fight, or something, then it'll get complicated for him.

