I’m not quite sure I understand what you mean by this.

You said,

if you could interview the same people before the something horrible [...] a lot of answers would be of the “he’s kind of a weirdo” type.

What I meant was: you should also consider the amount of cases where people said the same "he's kind of a weirdo", but that person did not go on to do something horrible. And also the amount of cases where people did not say it, and yet the person did something horrible. All three are necessary to calculating the strength of the evidence "people say he's kind of a weirdo" in favor of the hypothesis "he will do something horrible".

There's a common fallacy, which you may not have committed, but which your comment as written seemed to me to evoke. Logically it's equivalent to base rate neglect. In conversation, it's often triggered like this: a nontrivial value for P(A|B) is given, but the probability P(A|~B) is not mentioned. The listener doesn't have a good estimation of P(A) or P(B), and he doesn't think to ask; instead the high value of P(A|B) makes him think B is a good predictor of A, which is a fallacy. (Here, A=be called a weirdo, B=commit horrible deed.)

I’m pretty sure such effects are not linearly additive.

I'm not saying they're linear or otherwise well-behaved, but I'm pretty sure they are all generally additive in the sense that in any given combination, if you increase any of the factors, the total also increases.

Notice how media narratives tend to become either “I always knew he was up to no good” or “I’d never have thought he would do something like that”, but you almost never hear something in the middle. I’m even having trouble finding a concise wording for a middle case other than “meh”.

It may also be that "I'd never have thought he'd do that" is the middle, the default way in which people think of anyone they have no reason to specially suspect. After all, I don't expect a random stranger on the street to suddenly commit a horrible deed; why should I expect it more of an acquaintance unless there are concrete warning signs, which would make me say "I always knew he was up to no good".

The true opposite of "I always knew..." would on this view be like Harry's reaction to Hermione confessing attempted murder: "I don't believe she did it, it's a priori so improbable there must be another explanation or very special circumstances". However, when the media has concluded someone has committed a horrible deed and is morally culpable, of course you won't hear many people saying this to the media, even if they think so privately.