Conventional wisdom around here, as I understand it: He wanted to unify Magical Britain under a strong leader (namely himself, in the persona of David Monroe) and wipe out, or neutralize, or otherwise remove the danger posed by, the muggles. He wanted to do this because he was very, very frightened of dying, and he thought nuclear weapons (and potentially other things you might classify as "scientific overreaching") were likely to produce more destruction than he could escape. And probably also because he wanted power.