I did feel that Didact's turn towards being a douche was a bit disjointed and not very well explained, especially in regards to resorting to using the composer. He knew the composer was flawed; the process fractured the composed's minds creating abominations, a process he used on his own soldiers knowing what it would do to them. Then he targets what at the time was a completely helpless humanity. Sure, he blamed them for bringing the Flood to the Forerunner's doorstep, but realistically nothing short of the Arrays would have ever stopped them. Add to that, technically the Humans were protecting the Forerunner's and everything else from the Flood, by sterilizing infected planets. Didact at the time of resorting to the composer knew all of this. Nothing about his attitude towards Humanity is logical, unless of course he just plain went crazy.