One that makes him/her more than just a one-dimensional villain or antagonist, and one that you might even want to see redeemed if given the chance. No offense. I just wanted to know how they would look and sound like in your opinion and why?

There's no right or wrong way to do it. You just have to be able to relate to your audience on a certain level.

That all depends entirely on which audience you are addressing.

Which itself depends on the social circles you are in, the locations you advertise your work to, the timing of said work's release, and the type of audience those tactics are likely to draw.

There's no assurance you will get stuff like that right, and success can only be learned, not guaranteed.

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From good to evil, passing by neutral posture. He'd question the Mane Six, the Princesses, and others. He has common sense, yet cares for people. However, he can feel betrayed at some point when they stopped believing in him.

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They can have loved ones, treat their people well, and second-guess themselves.

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One idea is make an antagonist that is a foil for the protagonist(s); they narratively stand on opposite sides, representing two sides of the same coin. They may even have the same drive but a different end-goal. In "The Dark Knight", Batman and the Joker both want control over Gotham City and its inhabitants, which is the same goal, but the difference is their motivations: Batman wants to protect the people and preserve their goodness whereas the Joker wants to create total anarchy and prove that everyone is rotten deep down. Their struggle is ideology: hope vs. nihilism and order vs. chaos.

Or, alternatively, add in some moral ambiguity in order to make your audience reconsider the story's heroes and villains. In my head-canon series, King Sombra was an antagonist because he wanted to bring down the current Equestrian regime, which the Main Six serve. However, it is more out of feelings of revenge and personal loss rather than malice. Aside from having understandable motivations, he also brings up a point that maybe Princess Celestia isn't the pure, 100% paragon of virtue that she portrays herself as and that the heroes are only helping to enforce her flawed reign. This changes how the world is perceived, both in-universe and out by challenging the white-and-black status quo.

Closest thing to that is season 2 Discord.