I'm late to this thread. As a Fool (with a Morris dance team) I have an interest in Fools, jesters, clowns and the like.
As a 57 year old Englishman, I feel that the whole "scary clown" thing is a fairly recent development. As a kid, I associated clowns only with the circus, and circus clowns were regularly shown in comic books and stories as benign characters. If a clown turned out to be a Bad Guy, we were meant to be shocked: "How could that jolly clown turn out to be a bank robber?"
Clowning then moved out of the circus. We now have "party clowns" (perhaps they had them in the USA earlier than we did) and you also get the obligatory "balloon animal clown" at village fetes and shows, and people in clown suits collecting for charity, and so on.
These new clowns usually have no background in clowning. Many seem to feel that wearing the costume and make up is 9/10 of the battle.
Then, having vaguely read something once about the clown "subverting social conventions" a few of them take the costume, wig and face paint as a licence to behave in a manner that would not normally be socially acceptable. In the guise of the clown, they can approach strangers, and force their often poorly thought out attempts at humour onto them. It bugs people and can genuinely be creepy.
Meanwhile, there is a real thing that the smile of the clown is painted over the ordinary expression of the person. There is a horrible disjunct between the big colourful smile and what may be a sad or grumpy expression behind it. We are allowed (forced?) to get closer to clowns now than we did when they were confined to the circus ring, and the man beneath the makeup is more clearly visible.
Add to that the fact that in modern society we perhaps have less patience than people use to have with deliberately stylised performance: the set gestures for happy, surprised, angry, afraid, wistful, pensive, etc.
Of course modern entertainment has its own conventions: examples include the white hat/black hat thing in westerns; the Bad Guy in films having dark stubble; the master villain having a shaven head; the girl folding one leg behind her when she kisses the man at the end of the film. We sort of recognise these conventions and may ignore them or affectionately laugh at them, but we draw a distinction between that and the intrusive artificiality of the "this gesture means that emotion" approach of traditional clowning.
We thus end up with a situation in which many of the clowns we encounter are poorly rehearsed and possibly socially inadequate individuals, wearing fancy dress shop costumes, doing boring stuff with balloons, getting a little too close for social comfort, and having a normal face and expression beneath an expression drawn on in badly-applied face paint. This is not a fair description of all clowns, but it is a fair description of many that I meet either as a member of the public or when I'm performing at events with the Morris dancers.
This creates a feeling in many adults and some children that clowns are "odd" rather than "funny" and that clowns are irritating and therefore to be avoided. We have far more taboos about invasion of personal space (Covid 19 aside) these days, and we are more nervous of being picked out from the crowd. Certainly the English have become a race of cautious observers rather than participants.
So, if we have reached this point where the clown is generally seen as a Bad Thing, it is a small step to seeing him as a Bad Man.
Feed into that the trope of the evil clown from American horror films, and you get two effects: inadequate clowns trying to incorporate some aspect of "evil clown" into their personas; and audiences associating clowns with "evil".
Personally, and I've met a few, and watched many on YouTube, I think most clowns aren't evil, they're just boring.
However, watch someone like
Abner the Eccentric on Youtube and you will see someone with genuine skills and charisma, who uses some subtle gestures, little or no make up, an understated costume, and can be extremely entertaining.