Have him rescue Thaniel, then murder him, dump him with the Myconids before you enter the Shadowfell and once you're done in the Shadowfell, act 1 is cut off and you're completely rid of him (at least, his tent disappeared while Thaniel still stood there, not giving a damn Halsin was suddenly gone).
He'll give you experience which is more useful than his entire presence in act 3. The Myconids will use him in a way that he seems to enjoy, in the Underdark which he seems to love. That's a win/win to me. Or you murder him and just let Withers do whatever Withers does with dead allies. Maybe he'll be a mindless hireling for someone else somewhere in the world - it'd be an improvement. Either way, nobody else in the party gives a damn if you do this, because it's Halsin and nobody cares about Halsin.
You can also let him get kidnapped but that requires Lae'zel as active party member and it takes a while so he gets to go full creep mode, and if he gets kidnapped then I'm pretty sure Yenna will never cook delicious soup for you which means his kidnapping actively stops character development of a character more relevant than he himself is in his current state. After all, we know it's canon that Yenna gets adopted by Tav and <romance partner except poor Karlach who goes to Avernus and should probably not bring the kid along>.
Jokes aside, at least there are plenty of ways to get rid of him without angering anyone and still finishing his quest. And until he gets real content in act 3 I don't see a reason not to, since right now at best he adds nothing and at worst he adds.. well, we know what kind of inconsistencies he can add.
