To make it clear...you must read this...
Sparky : "More or less. Technically this installation's pulse has a maximum effective radius of 25,000 light-years. But once the others follow suit, this galaxy will be quite devoid of life-or at least any life with sufficient bio-mass to sustain the Flood...But you already knew that...I mean, how couldn't you?"
"We have followed outbreak-containment procedure to the letter...you were with me each step of the way as we managed this crisis..." "Why would you hesitate to do what you have already done?" "Last time you asked me, If it were my choice, would I do it?" Having had considerable time to ponder your query, my answer has not changed, there is no choice. We must activate the ring." " If you are unwilling to help, I will simply find another. Still I must have the Index. Give your construct to me, or I will be forced to take her from you." |Chief: That is not going to happen.| Sparky: "So be it. Save his head. Dispose of the rest."
At the end of Halo 4, you here the Didact speaking to Chief. But, is that really the UR-Didact? I think not, I know for a fact that that is Bornstellar speaking to him. Bornstellar probably awoke in Chiefs head after the Librarian had moddified his genetics.
Guilty Spark knew that Bornstellar was in Chiefs head from the end of Halo CE, and thats why he wanted Chiefs head, So that he could extract Bornstellar from Chief, like they did with Sparks when he was still a human named Chakas to get the Lord of Admirals out of him.