Question: "...How could someone like him with every advantage in the world go so bad..."
Answer: Because a life spent in show business from childhood on, is anything but an advantage...
It is closer to being a curse.
Such a person has not experienced so much as one moment of "normality" at any time in their life.
Everyone surrounding them at every moment is a huckster, liar, whore, deviant, sociopath, flesh-peddler, syncophant, crazed fan, or someone pretending to love them who is planning on stealing their assets.
Add to this environment whirlwind non-stop tours; a different major city every 24hrs, intense physical & emotional exhaustion, then on to the next city...
Then, toss in literally so much money that they have to hire attorneys and accountants just to keep their original attorneys and accountants from stealing every cent they make.
Result--life of abject ISOLATION.
Can't even so much as ride down the street to shop for groceries.
Can't go to a movie.
Can't go for a walk.
Can't go to a bar or singles place.
Can't walk into the bank.
Literally can't throw your own trash out back of your house, because photographers are hiding there to take a picture.
This life described above is what is considered SUCCESS in show business.
Still think it is "every advantage"?