well, the Didact does have a hatred for humans. He was a leader in the forerunner-human war, he lost all of his offspring in that war, and he was outraged when he discovered the manlte was meant for the humans. In the tenth terminal in halo CE evolved anniversary, 343 guilty spark is talking about the Didact at the end of it. He specifically says "and without the Librarian to temper his rage, those Reclaimers would almost prefer the flood". Humans are reclaimers, and this means the Didact is a threat to humanity. We can also assume he is very dangerous, because its pretty hard to prefer the flood over any other foe.
But i am not sure how the Didact would have had a human as an ally. I'm pretty sure it was before he knew that humanity was supposed to inherit the mantle, so this may have a factor in this.
Or i also have the idea that he wanted to use the humans for something, but he never got to do it. Thats probably why the promethens kept chief alive at the end of the campaign demo, becuase the Didact still needs a human for something we dont know