Anyway, I just recently had a little thought. As I recall there have been a lot of other Nerevarine candidates which all turned out false in the end. It's mostly assumed they are dead, however I had another thought.
Throughout the main game you encountered Dagoth Urs followers including other Dagoths. However I asked myself "wait a second, we had other people who appeared to be the Nerevarine and I doubt Dagoth Ur will make anyone an ascended follower" and got to a conclusion.
A lot of the other Dagoths are actually previous Nerevarine candidates.
They all got to a point where they contracted Corprus and for them it SEEMED like they would be affected less by the disease or in some cases like they where not affected at all. However, unknowing to them it still slowly worked in their bodies, somehow it just didn't mutate them as fast as other people.
Once they managed to make it close to Ur he simply "talked" to them and managed to convince them of his side. By that time they probably already fought with pains of the mutations and with Urs divine powers he could make the pain go away (or at least make it feel like that) which is a easy way to get them on his side. Plus he could control the mutation and make them ascend instead of become horrible out of control mutations.
If you look at the other Dagoths they all seem to be Dunmer, since Urs plan was to "liberate" Morrowind of outlanders this, plus the pains of the mutations, must have been rather convincing to them.
This could also explain how Sunder and Keeing got in Dagoth Urs hands, they where originally simply given to the Nerevarine candidates and couldn't be retrieved anymore.
Now I don't remember all details anymore but this is a little theory and, if you think of it, a bit disturbing as those guys you kill where your predecessors.