When an audience sees the evolution of a character whose ethics progressively spiral downward, they don't turn against the character. Instead, they remain loyal to him. especially when the antagonists concurrently get worse with the villain.
It's likely the result of a constant comparison with other characters. It shows the importance of how characters are framed.

Framing the villain

One study manipulated what characters looked like and measured audience perceptions. They hoped to find out if simple differences in appearance would be enough for viewers to perceive a character as a hero or villain.
The findings indicate that we judge based on comparisons and not because of using an objective standard of morality. Heroes were judged to be more heroic when they appeared after a villain, and villains were judged to be more villainous when they appeared after a hero.

Continued comparisons

Morality matters. People tend to like the good guys and dislike the bad guys.
In a new study, researchers suggest that we don't need to see behavior to make a distinction between the hero and the villain.

The hero

New research shows why people might feel good about characters who do bad things.

Framing the villain

What makes us root for the bad guy?

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What Makes a Superhero?

Immersion in fictional worlds allows us to own our dark side | Psyche Ideas

I wonder which villains the authors of this study are fond of.