Orks da best said:um humanity, or ancient humans did not push the flood out of the galaxy, and in either halo 4 plot the flood was following humanity for food or the plot before halo 4 came out that the flood randomly appeared in milky way, possiblely from another galaxy. In either case the forunners couldn't handle the flood, in halo 4 they were weaken by the conflict with the ancient humans, or were just plain unable to deal with them with anything short of mass geonicde in the story before halo 4.
So in a sense the Didact has a point with humanity leading the Flood to the Milky Way, as well as engaging with a war for what no reason really. Petty good reason to be hostile to a species that openly attacked you without good cause and lead another hostile life form to you. Though that is in the halo 4 plot, before that yea he would have little reason to hostile to humanity.
Yes, they did.
http://www.halopedia.org/Human-Flood_war
" The war destroyed countless human-populated systems and cost more than a third of humanity's population and significant military resources. Despite the setback of two massive wars ongoing at the same time, humanity managed to prevail over the Flood and still nearly prevailed over the Forerunners, as well."
And the flood were created as a weapon by the precursors to "test" humanity.
http://www.halopedia.org/Flood#Origins
"the surviving Precursors put into action a plan that would involve a kind of test that would come to the Milky Way galaxy... This test would serve as a means to eliminate the Forerunners, ensure greater peace and unity amongst the galaxy's inhabitants, and to test whether or not humanity was worthy of the Mantle. This test took the form of the Flood, an all-consuming collective parasite. Around this time, an ancient Gravemind was created from 12 or more unspecified creatures."
The humans lost several colonies in the war against the flood and attacked the forerunners in order to survive.
They weren't blameless or completely innocent, but they didn't just attack for no good reason.
http://www.halopedia.org/Human-Forerunner_War#Origins
"Driven to desperation after losing many of their colonies in their conflict against the Flood, humanity invaded a Forerunner-controlled sector of space and annihilated fifty defenseless systems in which the Forerunners had resettled other species. After the indigenous populations were eliminated, humanity replaced them with human colonies to strengthen their hold over their new territories. Humanity looked to forcibly take new worlds anywhere, including those inhabited by Forerunners.[10] However, not all of this seemingly irrational violence was driven by the will to expand; instead, human fleets were sterilizing planets with Flood infestations.[11]"