its happened to worse characters, but usually in more light toned stories, like Vegeta.
Any character who is shown to possess some kind of humanity is
capable of redemption.
A character like Vegeta was able to be redeemed once you had him begging Goku to avenge the Saiyan race as he lay dying, whereas prior it wasn't narratively possible since Vegeta hadn't shown any signs of caring for anyone or anything else. Once you humanise someone like that, it kind of becomes impossible for a writer to not make them morally grey at the very least because very rarely do people do evil acts without some form of moral rationalisation. Edit:
In the movie, "Look who's back," they took any possibility of Hitler being redeemed by taking away one of his most well-known "human" aspects: his love of animals. They do this by having him shoot a dog relatively early, thus removing any possibility of Hitler becoming "good" by modern standards. Ironically, Vegeta's humanising love for his people thus qualifying him for redemption would've qualified Hitler for redemption narratively because any show of "love," the most human and empathetic of emotions, puts your foot in the door for a redemption arc.
Spencer feels so weird to redeem because his show of humanity (adopting baby Grace) and redemption (Elpis) all occur within the span of 10 minutes. "Unconvincing redemptions" are a thing you can look up and receive a list of them because it's a semi-frequent occurrence in badly written stories. A somewhat recent example of this is Kylo Ren, where, similar to Spencer, hus show of humanity and subsequent redemption all occur within the span of 5 to 10 minutes.
People
can change, but you can't flip a switch and suddenly reorient someone's entire moral structure, which is the issue with Spencer's redemption because it feels like that. Nowhere in any other game did Spencer show any regret, remorse, or restitution—he was a crazy old man who wanted to make cool BOWs and become a god.
Thinking about it, I don't recall whether or not Resident Evil has even tried to do "morally grey" before, or a redemption arc. The most we see is good people unaware to the true extent of the evil they work for (Mercenaries in Resi 3, Ada Wong in general), mind control (Jill RE5, The Baker family RE7). Maybe Heisenberg in RE8, but he still wanted to be evil, just not evil under Mother Miran
der.