OK, full disclosure, Adolin has turned into my favorite character, and its not close.
Getting that out of the way, I'm crazy excited for the directions for Adolin to go in the next couple books. I agree with trev1972, I don't see him "going to the dark side." But I do see things getting really rough for him. I think books 3 and 4 will see him utterly broken emotionally, and eventually turned into a radiant. I was going to start a separate thread on Adolin's future, but I suppose there's no reason to have two Adolin threads. Here are my reasons.
1) Adolin has spent his entire life training to the greatest warrior in all of Alethkar, because he idolizes his father, the former greatest warrior in Alethkar. The irony is, that for just one short moment, Adolin succeeded. Additionally, his sense of protectiveness and comradery with those close to him is at least as strong as Kaladin, as shown by his willingness to sacrifice his own life in the peace negtiations with Eshonai, and his Jail Solidarity with Kaladin after the duel, Adolin just tries to cover it up, because it doesn't fit with his public persona. The only difference with his sense of protectiveness from Kaladin is that Kaladin protects out of honor and principle, and Adolin protects out of love and friendship. (Every time he has an open conversation with someone, it turns out that the big oaf is emotional as all get-out)
Now, at the end of the book, everyone Adolin loves and wants to protect - father, little brother, fiance, and best frenemy - have become radiants; they don't need his help. On page 1066 we literally have him sayng "What was he?" about himself. How does he define himself in a post-radiant world? How does he cope with his military irrelevance?
2) To me, this doesn't mean that he turns evil. The most central aspect of his character, to me, is his ability to make friends. He even turned Kaladin into his bro, which was basically a miracle. I didn't read much hate into his murder of Sadeas. Yes, I know he uses the term "irrevocably enraged," but that is how close quartered fights are. Try this passage on for size from 1067:
The sentiments I read out of that passage were sorrow, and duty. Yes, anger followed, but nothing hateful.
I do think however, that rough times are ahead for Adolin. Rather than becoming evil, I think we're about to see Adolin destroyed by a force much more powerful than voidbringers or radiants. Yes, I'm talking about SHALADIN
3)Over on the Shallan relationship thread, several people are posing incredibly well-supported arguments for why Shallan is on the verge of leaving Adolin for Kaladin. The arguments are too long to post here. My spin-off: We already have quotes from Adolin saying that what he wants most in life is for a relationship to work out. He just wants to love and be loved. Now, he's found a girl who is literally perfect for him in every way, except she's being drawn to his best frenemy. If Shaladin happens, it would break loverboy Adolin in profound ways, which leads to # 4.
4) A few spren have mentioned that only broken souls can be reforged into Radiants. That is why literally everyone around Adolin except the the blondey himself have become surgebnders. They've all been broken in some way. Adolin has not. He's had a somewhat easy life, which might be why love is the most important thing to him (Maslow's Hierarchy). A bunch of people have mentioned on the forums that Brandon implied that Adolin is a dustbringer. I have no idea if ths is true or just a dirty rumor. However, if true, Adolin would need to be broken before his spren could appear to him, but how do you break Adolin? The dude is a paragon of mental health.
The answer: You throw away his life's work (becoming the greatest warrior in all the land), and then you break his heart.
Edit: Props to carlos stormblessed also. he wrote a lot of the same steps, I just think it'll play out a little differently