Well, I have so simple and obvious theory, that knowing Brandon, it has to be wrong.
Odium says: We killed Honor. It hints, that he had help. Odium is afraid of Sazed, who has power of 2 shards and Stormfather says, that he is worried about exposing himself to Cultivation. So it seems, that Odium is unable to fight 1v2 shards, so he couldn't kill Honor, while Cultivation was on Roshar. And he freaks out, when he thinks he sees Honor, so it indicate he lost acces to his help. It would make plausible, that Ruin participated in splintering of Honor, since it both suits to his MO and not being able to help Odium again.