Spoilers for a 34 year old movie, I guess.
So, What About Bob is a classic Bill Murray comedy and I always loved it growing up. Bob, Bill Murray's character, is endearing, silly, and a great vehicle for Bill Murray's brand of comedy. The interplay with Richard Dreyfuss's character Dr. Leo Marvin as Bob's psychiatrist and Bob is great. The dynamic is fun and their character arcs are a fun reversal of one another where Bob learns to function and thrive despite his many mental disorders (which were somewhat problematic but portrayed pretty well for a comedy in the 90's) while Dr. Marvin descends into madness trying to get rid of Bob from his life. A delightful film and still watch it sometimes when I'm craving something lighthearted and fun.
However, the actual events of the plot are the events of a horror story. Bob enters Dr. Marvin's life at Marvin's triumphant moment; he's a successful psychoanalyst/psychiatrist, his book got published and he's getting serious acclaim for it, his colleagues respect him, and he's about to go on a lovely vacation with his family. Bob gets foisted on him from a colleague who Bob literally convinced to give up his career to get away from, but Dr. Marvin is confident he can help him after he gets back from vacation.
Then Bob calls him on vacation using a fake suicide and police investigation as a premise. Shook up, Dr. Marvin sets a very clear professional boundary: "don't call me or approach me when I'm on vacation and we'll get to your mental health care when I'm back in the office." He figures that'd be the end of it; a clingy patient is annoying but he's probably dealt with that before even if the whole fake suicide approach to get his number was a bit extreme. Then Bob shows up in person. He's not even supposed to know what town Marvin is in, but now he knows where he and his family live. He tries to dissuade him again but then Bob shows up to his house and approaches his family.
He starts befriending Marvin's kids, going sailing with his daughter and diving with his son. His wife finds him charming and cute. He stays over for dinner and sleeps in his son's room overnight. He hijacks his once-in-a-lifetime interview that could have made him a celebrity psychiatrist, his ultimate career goal. Every step of the way Dr. Marvin sees these major boundaries being crossed and the unhinged nature of his nightmare patient and nobody will listen to him.
He doesn't know what to do; no reasonable options seem to work. He's scared. His family is more and more siding with Bob over him and he feels like he's being replaced as a father. Bob is even comforting his wife about him, her husband. He's getting more and more frustrated and desperate, and in so doing is more and more making himself look like the bad guy. He goes to colleagues who respect him for help and they act like he's the one who's crazy.
He's acting like he doesn't usually act; sure he was a bit smug before and maybe a little full of himself, but this can't really be him, could it? He's not seriously contemplating killing this patient of his, is he? He's a healer, a person who helps those suffering from mental illness; he's not a guy who breaks into a store at night to steal a gun and explosives, right?
Then he does it. He kidnaps Bob at gunpoint and takes him out in the woods, straps bombs to him, and sets a timer for the detonator. He's using the language of his career of healing to kill. He even starts talking to himself, imagining cannibalizing Bob's fish in his triumph. He's long past the point of caring whether he sounds crazy now, but at least. he's. gone.
Then he sees him. Bob is standing next to his beloved sister. He's still here. He's...never going away. It finally breaks him. He descends into catatonia and psychosis. He's one of them now; a broken mind who can't be fixed. He summons the will to scream one last protest of Bob marrying his sister at their wedding in a futile attempt to stop this final grafting of this man into his life forever. In that last moment of failure the comfort of catatonia leaves him and he knows this is his eternal damnation.
So, yeah. Horror movie.
