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).

*bows head to superior* [spoiler]p.s. what happened to the other Diadact[/spoiler]

The IsoDidact, after firing the Halo Array from the Ark's Citadel, stayed on the Ark for a few years to help prepare the reseeding of the galaxy. He made a stop at Earth with Chant-to-Green make sure that the Homo floresiensis were happy with their new location when humanity was reseeded. After that, well, we don't know. He and the other Forerunners went on the Great Journey. It's widely believed they left the Milky Way, however there are sources that have clarified that there were no Forerunners left in just the Orion Arm. That lore is old though, so it's unknown how canonical it still is.

Ahhhhhh thanks again.