Caede started out as a good guy, but the more you learn from lorecards, the more it is clear that before being a guardian, he was a Clovis Bray killing machine. Before that human Caede is portrayed as a badguy.
Uldren started out as a badguy, but the lorecards paint his past in a far more positive light, plus Uldren can defer part of the blame to Riven.
In a sense, Uldren and Caede are the same:
Human Caede good -> Exo Caede Evil -> Guardian Caede Redemption and Death
Early Uldren good -> Destiny Plot Uldren bad -> Guardian Uldren now live
The only difference being that the player can say Caede past existence as a killer is water under the bridge, while Uldren's villainy was experienced first hand.
Which brings us to Uldren being in a position where he has hell to pay and lore cards suggest he is the man for the job.
In true Manga and Comic tradition, Uldren will have to redeem himself and just when you like him, that is when he really dies for shock value, completing his character arc.
For the same reason you want the Callus situation to get worse, revive Ghaul and watch the glory of him fighting the Pyramid ships. Remember, we also need a new speaker and probably all reinforcements Cabal and Fallen can send to Earth and make a stand.