Most of it was because he was rushed, he wasn't originally intended to be a companion, currently Halsin's personality seemed to be result of him being added last minute, with no proper direction to give him.
Honestly I heard that the most interesting thing about him was cut for some reason... .
He killed Isobel with the Glaive now named Sorrow and in turn caused Kethric to unleash the Shadow Curse upon the land.