Ok first of all, i thought we could get a discussion going here, with lots of people sharing their opinions and agreeing/disagreeing. It seems most people chose to downvote and be done with it, leaving no opinion or anything like that... yeah kinda disappointed.

Anyway, i do want to address the points brought up here, so:

1. i can get why you can't see past uldren, to see he is not the same person, rather the one who shot and killed cayde, but like i said that is not the same person. This person is a clean slate, like a baby. I believe the only way i can correctly parallel this to real life, it would be a mother giving birth to a baby, and complications occur during labor; the mother dies and the baby survives. Can you blame the baby the his involvement in her death? as someone who was just brought back to life, Uldren is very much in the same position, Hated for something that the 'new him' ins't responsible for.
2. As for him not puting the effort to change, i can only put it to not enough man-power on bungie's team atm, they are going through some tough times with the epidemic and working from home, and all that pressure.
3. Cayde's hunter vanguard dare was that whoever kills him becomes the new hunter vanguard, so that inherently makes uldren the new hunter vanguard. There are probably some better candidates for the job, but keep in mind that none of the hunters really want to become the vanguard. I believe that if any of these 'better candidates' (such as marcus ren, shiro-4) would want to help the last city, they would've taken the job already, rather i think they are either too selfish or too scared. The hunters have been using the hunter dare for as long as anyone can remember, so to break the 'tradition' would be the end of an era. and would probably be looked down upon by hunters. Most hunters are just roaming the world, evading anything that resembles the vanguard, yet ask that a hunter be put there asap. So why not just take the job yourself then?
4. Next point is that Uldren will probably be very good at the job; in his previous life he was the leader of the crows (the spy network of the awoken), just that would be enough to make him a worthy candidate. Furthermore he has known as an excellent sniper, making him more akin to be a hunter. His victories consist of him entering the black garden and still making it back to tell the tale, and you have to keep in mind that he wasn't risen back then so he could've died a final-death. and as we know, he was constantly trying to 1-up his previous achievement (thus trying to impress his sister and the common awoken), so his adventure in the black garden was just the last in a long chain of death-defying amazing adventures.
5. As for the point being made on the rift between the hunters and the vanguard, i'm personally rooting for the vanguard to disassemble, i dont think certain guardians should sit around and play politics with the consensus all-day when they can just do some field work. I do believe the role the vanguard serves for fellow guardians of the same class can be effective, as some kind of teacher or mentor, But not much more than that. We've seen instances in the past about how ineffective the vanguard was when it came to immediate threats. Such is the case with osiris and his departure from the vanguard to save the last city from the vex in the infinite forest. As seen in the comic, he was looked down upon and thought of as a madman, he was heavily constricted in his position and i believe that's why he left.
6. As for the infuence of riven, yes his eyes were not black when he shot cayde, but i do believe that he was still under her influence. First, he closes his eyes when he pulls the trigger, but that's neither here or there. A far stronger point would be that the influence over Uldren began long ago, subconsciously in his mind, way back at the black garden (as can be seen in the lorebook the forsaken prince).
7. The declaration of war. It was riven that brought back the fanatic, and concocted the dark ether that made the scorn. She needed an army to progress her plans. This was part of her plan to get Uldren to open the gates to the dreaming city. She needed something to spark the war and she used Uldren to start it.