There is no magic about the word "allegedly". "Rob allegedly killed Alice" simply means "Someone said that Rob killed Alice". It implies that the person reporting this is not claiming that it is true or false, but just reporting what someone else has said.
If in fact some person did say that, such a statement is true, and quite probably not defamatory. However, if the original claim is unreliable, and it is being repeated in a way that serves to lend it credibility, it could still be considered defamation by implication. If Rob has been charged, or accused by a credible source, or multiple such sources, reporting on that accusation will normally not itself be defamation.
Saying "I allege that Rob killed Alice" is just saying that "I claim that Rob killed Alice" and might well be defamatory if Rob did not in fact do so.
Again there is nothing magic about any particular word here. It is the overall meaning of the statement that makes it defamatory or not, as that meaning would be reasonably understood by readers or listeners. Any statement that will be understood as carrying a defamatory meaning may be an act of defamation.
There is also nothing magic about Rob having been formally charged. Such a charge is a fact that can be freely reported on. Widespread or credible claims that Rob is the killer are also facts, and can also be reported. But if the only source is an anonymous vid on YouTube, with no supporting evidence, that is not a credible claim. Reporting it in a way that makes it sound far more credible than it is may cause people to believe the false accusation, and thus be defamatory. Again the overall meaning of the statement is important.