I'm planning a solo run and my biggest hurdle is the inventory space issue. I'm running with a full party and I'm already feel that it's limited, it's going to be a lot more evident when I play with just CHARNAME.
Although Candlekeep is generally closed, there were still powerful individuals that could gain access. It is not inconceivable that CHARNAME met one such individual and was highly influenced by them, and they could even have acted as CHARNAME's mentor for a short period of time.
I roleplay Charname being a paladin as him/her trying to resist the Bhaal taint and murder lust by trying to uphold to a very strict set of rules to follow.