The Didact wishes to destroy us because of a previous war between humans and forerunners caused by Humans attacking a (flood infested) forerunner colony. This war resulted in the death of the Didact's children, fostering a dislike of humanity within him. This was capitalized upon by the flood, which utterly broke his mind during the forerunner-flood war. He then thought that humans would be best utilized as Promethans to "atone" while stopping the flood, but his efforts were thwarted by his wife, the librarian, hut put him into a cryptum in hopes that he could meditate and eventually recover to serve as a guide for humanity. However, the sum of a forerunner knowledge, something called the domain, was destroyed with the activation of Halo, thus consigning the Didact to 100,000 years of waiting, unable to do anything So he's pretty pissed