Totally agree with OP in every line. And we are not alone in this.
Sarevok and Viconia was my favourites from old games. My first playthrough of BG1-2 took place in 2014 and it was love in first sight. Poor graphics faded into the background very soon: the story was perfect.

I loved how strong Viconia was against odds of her life. She was hard, complicated, not like open book. And an outcast, not part of any cult of Shar. Even isolated from the story of Gorion�s Ward (mine was female) she was definitely not the woman we met in BG3.
And Sarevok. Honestly he is one of my most loved characters ever, so I can be biased. But his portrayal in BG3 make me really quit game and go get some air. Still can�t come to my senses actually. See, even in Baldur�s Gate 1, game from the distant 1998, villains wasn�t so cartoon and full black. Yes, that maybe was not the time of gray morale, but some digging could give interesting results.
BG3 Sarevok is quite the opposite of Sarevok from old games. He was fiery and desperate, but never a mindless brute (really! He tried, but even couldn�t start a slaughter without a plot! sweet chaotic evil). And Sarevok was never the follower of Bhaal. He. Was. Never. He wanted father�s power, he got under the influence of his blood, but never worshipped. I think Tamoko was the one who explains everything very accurately: she said Sarevok�s desire for power spring from hatred and resentment. Like our Bhaalspawn main character, he was torn apart between his divine heritage and humanity, but fate was more cruel to Sarevok, so he get what he get.
After resurrection in ToB he loses any spark of Bhaal�s power. And started to think deeper. Redeeming is optional, but what was exactly - Sarevok asked for new life. He wanted to be revived very much, to get "flesh and blood and bones" again. Returning to life didn�t bring him peace, his ending tells about it, but to fall into worshipping dead god? The one he lost every tie with? It just makes no sense. Hate to say this, but he better be dead, really.

I don�t know how to speak about �respect for the past� cause it looks like� have you ever read fanfiction? The ones where author treats badly with characters they don�t like? This. BG3 has many detailed evil characters with motivation and personality, so we can�t say it�s about moralism and plain �bad is bad�. Looks like these two is just very unloved by someone in Larian. It�s the only explanation I can think of.

And too sad to see liberation arcs of Astarion, Lae�zel, Shadowheart - they release from abuse, burden, control - but this right to be free was not given to Viconia and Sarevok.

Sorry for my English.