I think Sanderson is setting up Adolin breaking story arc. He needs to learn about his mother, he's showing signs of a lot of jealousy (He has able to handle it well so far but constantly bringing this up will affect him) and I'm sure his marriage with Shallan will have its own problems. I feel like all of this is set up will help revive Mayalaran. Everyone has different breaking points, so far in the story I believe we haven't seen Adolin truly broken. So far we've seen majority of the radiant cast broke to the point where they couldn't function.
Why do all radiants been to be broken? So Nahel bond could mend them? Connect the spirit webs? Or maybe radiants need to be broken and have enough will power to not give into those negative emotions , in return be strong enough to resist Odiums influence? Can Adolin resist it right now? I don't know, i'm not saying killing Sades was wrong but killing him with fluctuation of negative emotions strikes me as he isn't ready yet. But that's my opinion on it, I'm probably completely wrong anyway.
