I find I am using christian names mostly for antagonist as well. I guess if an antagonist starts off friendly then in speech he/she would be called thus. What is the general view about this.
Consistency. As long as you refer to the character by the same name to the reader, then it's all good. Characters can call each other whatever they want (a good example is a ranking officer. On duty, the dialogue surrounding him might involve his friend calling him by his first name, and other colleagues by his last name, and higher-ranking officers by his rank. But, whatever you refer to him as during the very first time you say "Brian replied." then it's Brian all the way down for you!
Depends. Hank if I want to give the reader a feeling of comfort and familiarity. Scorpio if I want more distance. Hank to his friends in dialogue. Mr. Scorpio to his underlings. Scorpio to his enemies. Hank Scorpio when I first introduce him.
Consistency, culture and respect. In many of the world it is considered rude to use the given name unless you're a close friend/family so keep that in mind in how they refer to others and others refer to them. In certain time periods people were frequently referred to by their title vs surname or given name. So for Arthur Wellesley, Duke of Wellington, unless the characters were family or close friends I likely would not have them call him Arthur or seven Wellesley, I would call him Wellington. I could see a villian(or a ruder main character) using a insulting diminutive of the characters name to drive home the point that they don't respect or fear them. For example, if their name is Richard, calling them 'little Dicky' to goad them. Especially if it was someone who used to be above them in terms of power/position/status to drive home the change.
Excellent points Mr. Cylenchar! Thank you. I have added these into my novel notes. Another revision I fear! Thanks all again.
I thought about this a bit later: just like using a diminutive can be insulting, doing the opposite can be used as well to come across as patronizing. For example, if the MC goes by Alex, the villian can go out of their way to call them Alexander. By doing this they can show they view the MC as childish or are humouring them.
Whatever floats thine boat, but remember that names have power. If you've ever used a hypocorism with an ex in front of their new beau, then you probably know what I'm talking about.
