If I thought that VoqAsh had bits of the real Ash's brain grafted into his own that might make me reconsider at least who I think he is.
Whoever he is though, whether he's culpable of Culbert's murder hasn't been resolved for me. We just have L'Rell's word that Voq is gone and they rushed through all scenes that dealt with the follow up. If he guilty? Is he sane? Who knows?
There was so little follow up I don't understand why they bothered to kill Culbert. If there had been follow up and consequences maybe I could see it (Although Trek's first gay couple on tv suffering the bury your dead trope so quickly is pretty disappointing), but since they briskly absolved VoqAsh of any responsibility and even rubbed Stamet's face in it by letting him see his partner's murderer roaming the halls free, I don't see where Culbert needed to die.
As I said in my previous post, I think AshVoq going free might have been politically motivated.
With Culber's death, I'm not going to lie: I wasn't super-attached to him although I like the character. I basically expect any supporting characters can die at any time.