This has been on my mind for a while. Would a Buddhist monk who spends his/her life devoting themselves to the well-being of others and being an overall compassionate person end up going to Hell for not believing in God?
Which God are we talking about here? The answer you get depends on the religion/denomination of the person you ask.
There may be alot of opinions when it comes to Buddhism, not as simple as saying Christian = Jesus, Catholic = Mother Mary, Muslim = Prophet, etc, because some say there's no god, but many would say there is (especially the older generations). Personally for me, there's no God in Buddhism, only an enlightened teacher. So would a monk go to Hell for not believing in God? First you should ask, which God? If it's gods of Buddhism, then theoretically, yes; Naraka, or buddhism hell.
if you find peace/happiness/life sense in some religion or way of think you're in the good way =)... if not keep searching your purpose there are many religions because everybody is unique and different, not everyone thinks the same.