Only the weak or the obstinate became corprus beasts, all others entered the path to enlightenment and empowerment. Sure, you might grow some tentacles from your face, but once everybody has tentacles I imagine it would have become an attractive feature.
Septim was a ruthless conqueror who'd kill even those closest to him if it served the mythic goal. If I remember correctly Dagoth Ur never betrayed or back-stabbed anybody, indeed, he's about the only powerful character in TES who hasn't...
He pretty much backstabbed Nerevar if I am right.
"The weak or the obstinate"? How do you know you are not one of those weak and obstinate? Or that your closest friend is one of these weak and obstinate? Or the person who you loves?
And seriously, there were like 15 tentacle-guys in the game. And hundreds of corprus beasts! Thats a big majority there being "weak and obstinate"! You seem to believe that there is only a few thousand guys who deserve to live a real life! The rest are to be slaves to your whims! That is just plain wrong! The weak and obstinate wont even have a chance of improving themselves! They are stuck there as slaves forever!
Tiber Septim was perhaps a ruthless conqueror but what eventually came from his actions was something good. Just because one person never lies or never betrays you doesnt mean he isnt evil.
Also, what proof do you have that Tiber Septim′s battlemage (because I guess that is who you are reffering to) wasnt plotting against Tiber? Or that he was crazy? Perhaps Tiber just used him because he was a powerful wizard?
Think of that.