Psychologically speaking, the reason why this monster killed his family has absolutely NOTHING to do with them at all. It didn't matter how much money they made, or what her job was, or how healthy and beautiful the girls were, or if anyone was having an affair or any of that. The reason he killed his family is 100% about HIM. Sure, he may have his "reasons" why he did it and may use them to justify his actions to others and himself. But the bottom line is that he is a cold blooded, evil, killer. He always has been. He didn't just wake up one day and do this. It has always been inside of himself.
To me, he is a classic case of a man living a fantasy-a true sociopath. They can seem nice, friendly, loving, whatever to make them acceptable to society. But on the inside, they know full well it is an act. They have no soul, no conscience and are simply an actor in the stage of life. They may think that if they portray this image that they will become that person they are portraying. But underneath - no. They know there is nothing inside of them. They know that they are a worthless human being. They know that everything they portray is a lie.
When that lie is about to be bust wide open (see Scott Peterson) they know the jig is up. So what do they do? They don't decide to kill themselves to get out of it. No. They are evil. They don't want to die. That isn't the answer. The answer is to remove what is about to reveal them and in many cases, that is their family.
If a true sociopath killer can get away with it, just think of the benefits! Poor man. His family disappeared one day. His whole family was taken from him. He gets the benefit of being a grieving husband and father just trying to make it out there on his own again. (see Drew Peterson.) No divorce. No name dragged through the mud. No revelation that he wasn't the perfect person he pretended to be. His secret would be safe and he could move on. These people in his life? He never loved them at all. The relationship didn't fall apart. He didn't fall apart. He NEVER loved them. If he had ever loved them he could have never done what he did. Period.