Manga/anime only here. I have a theory that maybe since the Chloe is the time traveling hero who sealed Veldora, and gave Shizue the mask, pretty much setting up Rimuru's journey. What if Leon is a failed Rimuru sent back in time by Chloe to help set up the next cycles Rimuru to hopefully succeed. And Leon is loner and sociopath because he grew more desperate and machiavellian to try to make his failing nation succeed.