If I remember the case correctly, he didn't make his home look uninhabited (that is nobody is living there) but as if the inhabitants had left (gone shopping etc.) to make it look attractive to burglars.
He then waited inside, armed with a gun, with the intent of shooting any burglars that might arrive. He shot the first burglar in the legs, and then proceeded to kill the unarmed and now defenseless burglar, who was lying injured on the ground and was in no position anymore to hurt him. He then did the same with a second burglar, shooting her in the legs, then shooting her multiple times, and when he found she was still alive, he shot her point blank in the face while she was lying on the ground.
You are asking the wrong questions. You are asking "is it illegal to remove a truck". It's not. What is illegal is to intentionally create a situation where you shoot people and try to claim "self defence". It can very well be argued that by luring burglars into your home with the intent to kill them, they are not actually illegal in that home, because you wanted them to be there. You can do many things that are each completely innocent but add up to a crime.
Actually, for everyone interested, I posted a question maybe last week or the week before whether you can be convicted for both first degree murder and second degree murder for killing a person, and it was exactly this case that inspired the question.
What should he have done? If he hadn't lured the burglars in, I believe the case would have still been a double murder, since he killed both unarmed teenagers when they were absolutely no threat. It might not have been first degree murder since it would not have been premeditated. But he intentionally lured them in, making it premeditated (first degree) murder.
If he had only injured them, the fact that he lured them in could very likely have made this an assault.
You asked: "Now that the burglars are in his house, what should he have done? " Well, he got himself into a dangerous situation. Remember, he was convicted for premeditated murder. So just before he shot the girl in the head, he should have instead put the gun away and called police and an ambulance. It would have been one murder instead of two. Just before he shot the boy, he should have put the gun away and called police and an ambulance. It would have been just attempted murder. When he heard the first person entering, he should have called the police and waited. When the burglar came in sight, he should not have shot and injured him.
It's a similar question to "if I try a bank robbery and there is an armed guard, what should I do". The only legal thing to do is to drop your weapon and wait to be arrested.
If an armed burglar had appeared instead of two unarmed teens, well, he would have put himself into a dangerous situation. Just as the burglar would have no right to shoot even if a home owner points a weapon at him, he had no right to shoot, no right to self defense, since he had intentionally created the situation. Tough shit. That's what you may get if you plan a murder.
Responding to some comments: @J.Chang Are you being serious? You are not allowed to make your house inviting to burglars, while waiting inside with the intent of killing them. Self defense only applies when a reasonable person would believe they are in danger. Reasonable persons don't think that a burglar comes in with the intent of blowing themselves up and taking the home owner with them. And no, you don't get to "assume the worst". Not when the worst is something no reasonable person would expect.
Thanks to Dale for pointing out that even for soldiers in a war situation, where different rules apply, deliberately killing a helpless enemy combatant is murder.