Do not forget there are two separate Didacts. Now, the Ur-Didact (Halo 4), he always disliked humanity to some extent. He had actually argued for our extinction after we were defeated in the human-Forerunner wars. The key thing though is that his mind was molested by a Gravemind, which severely distorted his perception of reality, and his ethics and morals were quickly eroded away in an attempt to save the Forerunners. This hatred was further compounded by him being imprisoned in solitary confinement (his Combat Cryptum) with only his own thoughts for 100,000 years (or if slipspace distortion is in effect, around 80,000 years).