I am glad that with Josephine Tey's novel, The Daughter of Time, (one of my all time favourites, the link leads to a Chandler's Ford Today review of mine) and the discovery of Richard III's body much more recently, there has been a lot of re-evaluation and the last son of York has a more sympathetic hearing these days. People are more open to the idea he might not have killed the Princes at all. Highly recommend reading Tey's novel and looking at the work of the Richard III Society.
Truth in fiction is as important as it is in life. If readers are going to willingly suspend disbelief for a while as they read what we have written, we have got to make our stories as realistic as possible to help that process along. It doesn't matter how fantastical or magical a setting the story has, the characters have to be real to make the whole thing work. So therefore the dilemmas they face, we've got to be able to identify with. There must be something in the characters that draws our sympathy.
It is a bit ironic that it is important to be truthful when portraying characters given that fiction in itself is something that is completely made up! But you still want the pretend world to be as real as possible for your readers while they're visiting it!