To sort out any confusion over the Didact in Halo 4, there are two Didacts - Bornstellar and the Ur-Didact.
If I remember correctly, The Bornstellar Didact was the one who went with a wounded Chakas to interrogate the captive. This was the one who began feeling sympathy for Chakas after all he had been put through. The Primordial said that Forerunners were never worthy to inherit the Mantle, which infuriated the Didact, who killed the captive, who also said there was no cure for the Flood.
The Ur-Didact, after his trial with Faber, was sent to a Flood-Infested planet, left to die.
So that's what the two Didacts were doing before all the confusing stuff happened.
One of the Didacts started coming up with ways to stop the Flood, advancing into a new form which still was not immune. His Prometheans volunteered to be Composed as the Didact's new form was immune to the Composer. Upon seeing that this worked against the Flood, he began using the Composer on humans, which the Librarian considered genocide and locked him away in a Cryptum. This is Halo 4 Didact.
Halo 4 Didact could be Bornstellar Didact for several reasons:
-He was the one who was told that Forerunners were not meant for the Mantle. Alternatively, this could have stopped him from trying to attain the Mantle.
-He was also told there was no cure for the Flood, which could have compelled him towards finding a solution. Alternatively, it could have deterred him from it and made him see activating the rings as the only solution.
Halo 4 Didact could be Ur-Didact for several reasons:
-This was the Didact who went to war with the humans long ago, and holds a personal grudge against them.
-This Didact was not told about not being worthy for the Mantle, so he could still think that Forerunners are the rightful holders of it.
-In the Silentium blurb, it describes the Ur-Didact or the Librarian making an "ultimate sacrifice" or something. This COULD be the Composer, however, it could also be activating the rings. It could also be the Librarian locking the Didact away in a Cryptum.
One of the Didacts tried to stop the Librarian from trying to save all the sentient species, but the Librarian eventually became stranded on Earth. This Didact saw no choice but to acitivate the rings. This could have been Ur-Didact, making his "ultimate sacrifice", or it could be Bornstellar. This Didact is NOT the Halo 4 Didact.
This is probably written in a confusing manner, however this information, plus what we will find out in Silentium, will probably help us deduce which Didact we faced in Halo 4.
If the Didact we see in the Terminals is the same one throughout, then it is DEFINITELY the Ur-Didact, as he was the only one who fought the humans. However, there is a chance that in the middle it switches to the point of view of Bornstellar Didact.
I guess we'll have to wait for Silentium for us to find out. Hopefully it'll expand on the Didact a bit more and further explain his motives. Personally, I liked him, probably because of his Harbinger voice.