This is an issue with the presentation of darker-skinned people in fiction. While you don't have to be a POC to have a valid opinion on the matter, it's absurd to suggest shutting them out of the conversation, or at least their opinion specifically as a POC, because they're the one's most affected by this issue.
Now in the second paragraph of the second fragment of your quote, you've made the assumption that there's a "lighter, non corrupted form" which existed in the first place, even if its not seen. That's not a valid assumption, as you can imagine a dark-skinned character doing something evil...and their skin colour not changing colour to reflect this. Unless you're implying that there's some external corruption force at work always, and even if this was the case, darkening of the skin need not be an indicator of it.
In other words, a "lighter and nicer" form does not exist by necessity, and must be presupposed, so you can create a villanous character that is always dark skinned, without running into the "darkening of skin" issue, as there's no reason that their skin wasn't always the colour that it is.
Essentially, the problem here must be stressed. In this thread, we're dealing with characters being given darkened skin to reflect a more evil version of them. Thus we have an issue of comparison - darker skin = more evil, which is both racist and colourist, due to applying negative stereotypes to a people simply because of the colour of their skin. There's no problem with having a dark-skinned villain who's always dark-skinned, as then their villainy is a feature of their personality - not their skin colour. Of course, you could run into a comparative problem here if dark-skinned characters were only, or overwhelmingly, villains - as then dark-skinned characters are more likely to be villains, and it's implicit that that's a feature of their race or colour, and you run into issues of colourism and racism again. The key point here is comparison, whether you have a change in skin colour, or an imbalance in representation.
