I always got the impression Russell had the idea that he could pluck out what he thought was the best principles and concepts from the bib,e and other ancient texts, other religions, faiths and groups (including secular and fringe), various schools of thought etc from various sources and cobble them all together to make something unique - a super belief.
He possibly (?) started out in a noble search for ... answers ? But as is common with charismatic leaders with persuasive rhetoric people placed him on a pedastal and it went to his head.
No one can say what his true motive and intention was except him, but if he was a charlatan or narcissist he would just lie and the people around him would construct a santized history of the man.