I really don't have much of any issue with Adolin killing Sadeas; by the end of the book it was painfully clear to Adolin that nothing was going to change. Even with everything Dalinar had said/seen coming true, Sadeas was still going to try and mess it up for everyone. At that point it's essentially putting down a mad dog for the betterment of the entire race.
Is it going to come back and haunt Adolin? Yeah probably, but as long as the next book isn't all stupid love triangle I'm betting he'll pull through it.