[SPOILERS] Why ruin this good film with the stupid twist at the end?

Vincent D'Onorfino was absolutely brilliant, so were the young and older kid who played Rabbit. The atmosphere, cinematography and premise were fantastic, and a very interesting and sort of credible story. I loved the way Bob was starting to see Rabbit as his own son and gave him some of his "love" (I mean to us he's twisted beyond comprehension but in his mind he showed Rabbit love he has never showed anyone before). Also when Rabbit finally kills Bob you can tell he feels bad he did, grabbing his hair and crying over his death, it was the only person he knew and took care of him for the last 10 years or so, Stockholm Syndrome on steroids.

And then:

He goes his Dad's new house and we know what happens, blablabla he set it all up. WHY WOULD WE EVER BELIEVE THIS!? First: Bob didn't kill for money, he would've done it for kicks anyway. But the main thing; why in god's name would the dad want to get rid of his pretty wife and son for absolutely no reason given and pay his psycho brother for it!?

It absolutely ruined the movie for me, what a shame it went from a solid 7.5 to a 5. Anyone else agrees?