I remember someone telling me that Stranger Things is about redemption, however, not every character got a redemption arc and I can name a few who did not.
Tommy Hagan
Carol Perkins
Agent Connie Frazier
Larry Kline
Grigori
Tom Holloway
Bruce Lowe
Ozerov
The scumbag and sleaze (Hargrove and Byers, beyond redemption)
Lt. Col. Jackass Sullivan (way beyond redemption, although he may appear S5)
Warden Melnikov (way beyond redemption)
Jason Carver
Yes, Vecna himself was also offered the chance, but he dropped the ball on it.