Why Kiss Plum? Why Dump Harry's Body?

Michael is completely rational guy until the last 15 minutes of the film, and then he suddenly makes these two extraordinarily random choices. Kissing Plum, that's almost forgivable. It was random, but he was going off the deep end with the book and his parents and all. Also, helping with Plum's body, I'd do exactly the same thing: protect you family by going along with the psycho until he's out of your f'ing house, then call the police. He decides not to leave, he decides to kill them too? OK. You kill him and THEN call the police.
What's not forgivable is choosing to dispose of the body of a man you just killed in self defense (defense of your family), thereby making yourself look very guilty if anyone comes looking for him. And they would. Are we to assume that neither Harry not Plum has any other friends or family? Also, he crashed his car. Police would investigate the accident.
By dumping his body in the well with Plum's, it looks like he killed them both. There wasn't even a shot where he's even thinking about phoning the cops. Nothing. He acted like he was guilty. No reason why.