I think David was at his worse when [spoiler] he locked poor Vicki up in the abandoned wing of the house.
I honestly believe he would have let her die in there. I wanted to scream at Elizabeth for gullibly falling for his lie that he didnt know where Vicki was...by this time she should have known not to take him at face value. [/spoiler]
So glad he eventually straighten up to be a more decent, albeit still strange, kid.