Is It Defamatory to Call Kyle Rittenhouse—or Anyone Acquitted of Murder—a "Murderer"?

Generally no.

False factual allegations about someone may well be libelous, but opinions are not. Is saying "Kyle Rittenhouse is a murderer" or "O.J. Simpson is a murderer" a factual assertion or a statement of opinion?

It depends on whether the statement is reasonably understood as (1) implying that the speaker knows undisclosed, unpublicized facts that implicate the target (potentially actionable), or (2) expressing the speaker's opinion about the facts that had been publicly discussed (not actionable). For instance, consider two more detailed statements:

1. "I had a conversation with Rittenhouse yesterday, and he told me a lot about what happened. The man is a murderer." Actionable factual assertion (which is to say that it could lead to liability).
2. "I've watched a lot of coverage of the case, and the jury got it wrong. The man is a murderer." Nonactionable opinion (which is to say that, as a matter of law, it's generally not libel).

The question is whether, in context, the bare statement "Rittenhouse is a murderer" implies what is set forth in (1) above or what is set forth in (2). Generally speaking, I suspect that most statements of the "Rittenhouse is a murderer" variety would be opinions, because listeners wouldn't think the speaker has any special knowledge beyond what we've seen in the news.

If you want an example, check out Gisel v. Clear Channel Communications, Inc. (N.Y. App. Div. 2012):

Plaintiffs … [sued] based on statements made by defendant Robert Lonsberry, the host of a radio talk show that aired on a station owned by defendant Clear Channel Communications, Inc. The statements at issue were made during an on-air discussion that [Robert] Lonsberry had with … Jacqueline Inzinga the day after her brother, John Gisel …, was acquitted of criminally negligent homicide for fatally shooting a man in a hunting accident. According to plaintiffs, Lonsberry asked Inzinga "how it felt to have a brother who was 'a cold-blooded murderer' " and whether plaintiff "'put a notch in the stock of his gun as he kills people?,'" and Lonsberry told Inzinga "that the hunting incident could not have been an accident…." …

[E]ach of Lonsberry's statements at issue constituted a nonactionable expression of pure opinion…. Because Lonsberry's statements were based on facts that were widely reported by Western New York media outlets and were known to his listeners, it cannot be said that his statements were based on undisclosed facts….

Further, the context in which the statements were made supports the conclusion that a reasonable listener would not have thought that Lonsberry was stating facts. Lonsberry's show used a call-in format and generally provided a forum for public debate on newsworthy topics, and his statements were made during an on-air debate with his listeners regarding plaintiff's culpability and whether the jury had properly acquitted plaintiff. Lonsberry had engaged his listeners in similar debates regarding plaintiff's culpability on several previous occasions. In addition, some of Lonsberry's callers used "harsh and intemperate language," and the tone of Lonsberry's statements was obviously intended to be caustic and confrontational, rather than factual. We therefore conclude that defendants established their entitlement to judgment as a matter of law that the statements in question were "expression[s] of [pure] opinion [that were] not actionable" ….

Illinois law follows this distinction between statements that sufficiently imply the existence of undisclosed facts (which may be actionable) and statements that don't do so and are thus seen as opinion based on widely-discussed facts (and therefore not actionable). Because Rittenhouse lives in Illinois, it is Illinois law that would likely apply to a libel claim.

Of course, more specific factual assertions may well be libelous, if they are false and tend to damage his reputation. I'm just talking here about claims that he's a murderer.