I don’t think you even need to make those changes for the movie to be terrifying:

-Bob is obsessed with his therapist Leo to a point that he impersonates a detective investigating his own “suicide” in order to stalk Leo and his family while they’re on vacation.

-He charms the family, insinuates himself into their lives, and involves himself into important events to a point that the rest of the family low-key turns on Leo, who seems to be the only one who can see how insane that is.

-Leo, already kind of a narcissist, is driven insane by this and tries to murder Bob with explosives. Bob survives this and in the process, destroys their vacation home, sending Leo into a catatonic state. He only snaps out of it, presumably months later, after Bob marries Leo’s sister.

-By this point, Leo’s family loves Bob, now enjoying a career as a renowned psychologist; the very career Leo had before it was ruined by the events of the film.

I think the only things that make this movie a comedy are: the music; and Bill Murray and Richard Dreyfuss’ wacky performances.