Something I didn't get (Halo 4 SPOILERS)

First of all, I’m not a reader of the books (nor do I care to be), so I’m not 100% informed on these things.

In the Halo 4 terminals, we see the Librarian approach the Didact and shoot him, and then presumably imprisoning him on Requiem, but in the Halo 3 terminals, the Didact reluctantly activated Halo at the end of the Flood war and killed the Librarian on Earth. How is this contradiction explained?

There are two Didacts.

So I’ve heard. Bornstellar and Ur-Didact. So your saying one Didact activated Halo and one was imprisoned on Requiem?

Why were they both romantically involved with the Librarian then?

> So I’ve heard. Bornstellar and Ur-Didact. So your saying one Didact activated Halo and one was imprisoned on Requiem?
>
> Why were they both romantically involved with the Librarian then?

Basically, Didact-Bornstellar is a bit different due to the Bornstellar origin but basically the same.

Bornstellar mutated with Didact genes, as well as some life-worker. After the mutation, he started to become more and more like the Ur-Didact, sharing memories and so forth. Bornstellar has more of a preference towards humans than the Ur-Didact though, so they’re still slightly different. Nevertheless, the reason why both care for the librarian is because they’re almost the same exact person.