If someone has actually alleged that Rob killed Alice, then the statement is true and therefore it cannot be defamatory. But if nobody has in fact accused Rob of killing Alice, then the statement is false. It also damages Rob's reputation, so it is defamatory.
If the speaker wants to express an opinion about whether Rob killed Alice, "allegedly" isn't the right word. It's also worth noting that "allegedly" isn't the same thing as "I allege."
News organizations use "allegedly" both to avoid the possibility of defaming the accused and to avoid prejudicing potential jurors. Before anyone is charged, it may be more precise to describe suspects as being suspected of committing the crime rather than as allegedly having committed it.