Uldren is actually pretty badass. Everyone in the Reef has reason to absolutely hate the Vanguard though, which is one of the reasons I wish we could choose a faction to play in this game.
One theory amongst many:
Nutshell. After Oryx Vs the Reef Fleet and Mara Sov's apparent death, Uldren crashed on mars. His whole group, the Crows, started trying to find him, as well as any fragment of the Queen they could (This last part is theory).
Uldren knew the only people he could count on were the fallen however, because he could beat them at their own game, and they also hated the vanguard as much as the reef does.
So he set himself up to take over the biggest house of Fallen he could, the House of kings. He set a really elaborate trap over weeks, and then allowed himself to be tortured for days/ weeks, knowing that eventually when they felt they had broken him, they would take him before the Kell.
They did, and the Kell of the Kings kneeled to him, which caused the others to do the same. The lore is spotty why the Kell did this, with some theories about a fragment of Mara being in Uldren, or even the Traveller, or just because Uldren was basically Conan with a spaceship and a gun. Dude(Uldren) aint bowing to shit, but he is going to do everything it takes to find his sister, and protect his people.
Mara Sov is basically the Traveller for the Reef. Uldren knows his people can't survive without her.
So yea, keep in mind the vanguard screwed the reef over, multiple times. The reef is denied the Light of the Traveller, they are denied ghosts and resurrection, they are denied anything recognizable as a safe home, they are constantly fighting everyone and everything, right up until the player walks in and is all "Hey Bros, I need a gate lord eye ball, cause a scary vex flower patch is bad!"
Two things that i think should be being asked in all this:
Where is Savathun? (Is she influencing Uldren, or even the Vanguard?
Where is Eris Morn? IDC what anyone says, this lady is a Hive creature now.
It would be really funny, and a nice story, to see Uldren go through all this shit, further the divide between Reef and Vanguard, only to pull his sister back from the void, and have it be Savathun instead.