I was always impressed with the development of David's character, per his behavior. Here is a kid with definite problems, doing everything from terrorizing his governess to attempting to murder his father. And yet, he never came off as a monster (despite what his father, Roger, said). He could've easily had been a Rhoda from The Bad Seed, but one always felt sympathy for him, no matter how nasty he became (although one could be tempted to take him out behind the woodshed on occasion). Through the writing and the plots, it was clear that he had reasons beyond his control for being rather disturbed, and even when he was up to his worst, he had pangs of conscience and even regret. To me, he was one of the most complex characters in the series.
Gerard