I'd probably rewrite the sentence as: "He's very insecure at the beginning of the story, but he is getting more and more confident. That's why is 'rules the world' at the end of the story". The difference, and it's very small, between "in the beginning ..." and "at the beginning ..." is that 'in' gives the sense of progression, whilst 'at' is a single moment in time. Hope this helps.