It's impossible to know how much his Jehovah's Witness upbringing contributed to his issues, it's probably a mixture of both his his upbringing and his bi-polar condition. If he was bi-polar it's likely he would have had issues no matter what his upbringing, but being raised in a doomsday cult can't have helped any that's for sure. The Watchtower engages in very extreme, black and white thinking, not a good thing for people with mental health issues. Perhaps early intervention with a good diagnosis and treatment could have helped, but of course that is discouraged in the religion. We will probably never know what could have prevented this tragedy if anything.
I worked with a woman who had this condition, it was heartbreaking watching her decent into madness. She become more and more manic, decorating her cubicle with plants pictures and rugs, just stuff everywhere, then she photocopied all her poetry, diary, doodles and musings of the last twenty years and gave a copy to everyone in the building, over 100 people, then sent a long email to everyone she knew, including higher ups, listing the many problems of her team and the many failings of her manager. She also had an affair with a married man and nearly destroyed his marriage. It was hard seeing a smart, beautiful woman fall apart, she finally went into depression and never came back.