Unlike Most other NPC's, Solaire never went hollow. The game implies that in order to permanently kill an undead they have to go hollow first, Solaire didn't. The same things happens to others if you kill them before they hollow. They just disappear from the game, when they should have re-spawned. Solaire really should be back at the last bonfire, him not being there is just story-gameplay segregation.
I'm also not sure if hallows are actually killable. If they aren't, then NPC hallows should just rewspawn.
I'm also not sure if hallows are actually killable. If they aren't, then NPC hallows should just rewspawn.