... because they had to make him distinguishable from normal Kirk but hadn't yet settled on the convention of just giving him a goatee?

Queercoding villains to make them seem dangerous and deviant to the people of the time (and those that are still stuck in that time). Admittedly, the people making that decision probably weren't conscious of that being why they thought eyeliner made him look villainous.

Gives him dark eyes, as opposed to the bright/spotlit eyes on good Kirk? So you can immediately tell there's a difference between the two.

Idk why but my shitpost-ridden brain instantly went "wait... d-did they make Kirk hot?"

Depending on your taste/sexuality, they arguably do that most of the time. The man seems to have shirts made of tissue paper.

Also this doggo is absolutely adorable in that costume

A large part of how we read emotions in other people is by looking at subtleties in their eyes. How open or closed, changes in shape that give away if they're telling the truth, etc.

In media, they often use dark makeup around the eyes, a shadow, or something like dark sunglasses as shorthand to show that a character is mysterious and hard to read, therefore possibly dangerous.

Probably the same reason that they make vampires sexy - the allure of the forbidden. Mr. Evil Kirk is the poisoned fruit.

Btw that sexual assault scene is even more fucked up when you learn that Grace Lee Whitney was sexually assaulted by an unnamed executive associated with the series...

So you know he's evil without a goatee.

You gotta pay the troll toll

I'm imagining William Shatner stumbling on this thread and having a fit of villanous laughter.

...cause he's F-A-B-U-LOUS!

He's going for gasps