Why?

CONTAINS SPOILERS
I am not a horror movie newbie by any means and I believe that I'm intelligent enough to follow a simple movie plot but...
Why did "John" do all of this? Was it his family that was killed in the accident? How did he pull off the physical symptoms the wife and daughter were experiencing? Was he secretly poisoning them somehow so their blood chemistry was whacked out, therefore causing the symptoms?
Why kill the child? (even though her constant whining and crying made me want to do it a couple of times)I realize now that the first couple of minutes of the film showing the daughter calling for her Mom are actually what happens to her near the end, but WTF?
Why was the Alex charged with a crime when he was defending himself and his family?
The movie wasn't anywhere near the worst I've seen, and I don't need a movie plot spelled out for me, but serously, this had some major flaws.

Which way you want to go depends entirely upon where you want to get to