Using Real people in Novels
There are sometimes good reasons for using real people in novels. Some novels are deliberately fictionalised versions of the lives of real people. If the available biographical knowledge is sparse, or has large gaps of time about which very little is known, that seems legitimate as long as the author is not claiming it is as accurate detailed biography. If the person is alive at the time of writing it could be damaging to present false information as true, to the subject, others around them or the author who might be sued!
In historical novels in particular it is quite common and legitimate to mention real characters of the period to lend authenticity to the story. It would be quite difficult to set a story in Victorian England and pretend that someone else was on the throne!
Even Shakespeare did it using past kings of England, Julius Caesar and Antony and Cleopatra, though he was careful not to upset living relatives in high places, such as King James when he wrote Macbeth!
If you include real characters who are well known it is more credible if you keep them true to their known attitudes and beliefs.
In his Roger Brook series of novels Dennis Wheatley includes real characters, who interact with the main, fictional characters, such as Prime Minister Pitt, Marie Antoinette and even Catherine the Great of Russia. Roger Brook was a spy, a double agent working in England and France. The books start before the Napoleonic era and go through to 1815. The historical detail is well researched and the real personages are kept ‘in character’ as far as possible. Similarly another of his heroes, Gregory Sallust, interacts with real people alive during World War II, including Hitler!
It seems less acceptable to use real people but to present them with different personalities. If there is real debate about whether someone was evil or misunderstood then you might want to present one view or the other. But there must be some evidence for the way the person is portrayed or there is no point in choosing to use a real person. It would make more sense to use a fictional character, even if you want to use a real person as a model and give your character some of their traits.
Information is so easily available now with on line access so it can only be confusing if a book contradicts historical fact or evidence (unless it is deliberately a ‘parallel universe’ scenario).