Samyaza operated off celestial instinct. Because he's a celestial he's a higher class and different species than man. His natural thoughts and ways were based upon him being a son of heaven. So he was in a perfect position to direct man towards God and enforce godliness through preaching, teaching, and executing judgement. But being a celestial- literally the real life equivalent of Marvel Comics' Beyonder pre retcon was not enough to prevent him from falling into temptation and changing his history and the history of mankind itself.