Some things I need to get off my chest, based on reading about and discussing history on various online forums and Facebook groups, and certain articles and books.
1) You shall remember that people who lived hundreds of years ago were complex human beings every bit as complex and human as we are, who had families, and feelings, and human dignity, and that therefore you should write about them with respect, in the same way that you would wish writers to treat the memory of you and your loved ones with respect decades or centuries hence. You will not laugh or sneer or gloat at their painful deaths and suffering, or claim that they deserved everything they got, or express a wish that they'd suffered even more, or call them vile names. If you wouldn't want someone in the future to make light of tragic events which have befallen you and your loved ones, or to depict your beloved father as a callously neglectful parent or not in fact your biological father thanks to your mother's cheating on him, or your kind and wonderful husband as a spineless snivelling coward who frequently beat you up and forced himself on you, or your daughter as a cold-blooded child killer - and if it would make you angry and upset if anyone wrote things like this about your favourite historical person - then you should think twice about inventing such calumnies about other people merely because you don't like them or because they were an enemy of your favourite historical person.
2) You shall remember that accusing someone of a horrible crime such as murder, rape, child abuse, violent assault or torture is a serious allegation which should not be made without real, actual evidence. This is no less true merely because the person you are accusing lived 500 or 700 years ago, and lame so-called justifications such as "s/he was an unpleasant person who might have done such a thing" or "s/he had a motive to commit the crime, in my opinion" or meaningless rhetorical questions and mealy-mouthed statements such as "it is not beyond the bounds of possibility" that s/he committed the crime are insufficient. A motive, or what you with the benefit of more than half a millennium's hindsight perceive to be a motive, does not in itself constitute evidence. A wish to point the finger at your favourite historical person's enemies rather than him/her does not in itself constitute evidence. A wish to portray your favourite historical person as a long-suffering victim to arouse your audience's sympathies for him/her does not in itself constitute evidence.
3) You shall remember that complaining about your favourite historical person being unfairly maligned by history, while enthusiastically maligning his/her enemies for all you're worth, looks hypocritical.
(I have been wondering whether I myself am somewhat guilty of this one, as I do sometimes jokingly refer to Roger Mortimer as 'Le Manly Wodge' or similar, which is pretty snide of me. Having said that though, my aim is to take the mickey out of bizarre modern statements about his sexuality such as Alison Weir's, and the assumption that his 'unequivocal heterosexuality' made him stronger, more virile, more manly, generally just better than Edward II not because of his abilities but simply by virtue of who he was sexually and romantically attracted to. My intention is to point up bigotry and stereotypes, and I do not in any way mean to be cruel or mocking about Roger himself - just about the way some people in the twenty-first century choose to depict him. I don't dislike Roger at all; he was an extremely able and courageous man and I find much to like and admire about him. Same with Robert Bruce, or Isabella for that matter, and I really don't see why I need to dislike and spit venom at people who were in some way Edward II's enemies. For sure I'd never make up the kind of hateful, hurtful slurs about them which certain Isabella fans have invented to throw at Edward.)
4) You shall remember that your favourite historical person's enemies were complex, multi-dimensional human beings too and deserve to be acknowledged as such, rather than as cardboard cut-out evil villains devoid of any humanity. Depicting them as cruel to animals, or attracted to little boys, or sadistic rapists, is a ridiculously unsubtle and obvious way to make them unsympathetic to your readers. You shall also remember that however much you like your favourite historical person, s/he was a human being and thus had character flaws and made mistakes like every other human being who has ever lived, and that depicting him/her as impossibly saintly and perfect looks kind of silly. And also strips them of their humanity.
5) Unless you're twelve, you shall remember that there is no need to divide historical people into 'teams' or 'sides' and hurl abuse at the other 'team' or people who like them.
6) If you're discussing history online and make a surprising or implausible statement, such as claiming that it was treason to refuse to have sex with the king of England in the sixteenth century, you shall remember that it is entirely reasonable to be asked for a primary source to back up your statement. This is not a reason to accuse people of rudeness and bullying and to get all huffy and offended.
7) You shall remember that modern historical novels, however well-researched, well-written and enjoyable, do not count as primary sources. Responding to a request to provide a source for a statement you've made about a historical person with "Historical Novelist X depicted him this way" does not actually answer the question. You should also bear in mind that merely because something has appeared in print in a historical novel does not automatically mean that it has a basis in fact, and you should check before repeating it as though it certainly does. This is how historical myths get started, and once established, they're damn hard to shake.
8) You shall remember that familial, societal and marital norms of the Middle Ages were different to ours, and refrain from referring to women as "helpless pawns" when their marriages are arranged by their (cruel, heartless, callous, uncaring...) fathers. You shall remember that having your royal or noble heroine wail "But I don't love him!" when informed of her impending marriage to a king or nobleman is by now a tedious cliché. You will not assume that a medieval king must have been an uncaring neglectful father because he didn't live in a nuclear family arrangement with his children. You will remember that, contrary to what you might assume, depicting Isabella of France as being willing to take a lover at the age of sixteen and foist a child of non-royal blood onto the English throne is an insult to her, not a compliment.
9) You shall remember that depicting women as all of a sudden no longer possessing their own agency, becoming the proverbial "helpless pawns" and coming under the total control of nasty unscrupulous men whenever they do things you don't approve of, when two pages earlier you were applauding their independence of action and thought as they did noble and good things, is as patronising and paternalistic as the 'sexual prejudices' of previous centuries you're decrying. Repeat to yourself until it sinks in: Adult women are responsible for their own actions, good or bad, just as much as men are.
10) If you wouldn't refer to Roger Mortimer as Isabella of France's 'straight lover', to Alice Perrers as Edward III's 'female lover', or to John of Gaunt's 'heterosexual relationship' with Katherine Swynford - and of course you wouldn't - then you shall remember that there is no reason to call Piers Gaveston or Hugh Despenser Edward II's 'gay lover' or to talk about their 'homosexual relationship'. Merely 'lover' and 'relationship' or 'sexual relationship' will suffice; it will be readily apparent to your reader that Edward, Piers and Hugh were all men and that their relationships were therefore evidently same-sex. Furthermore, you shall remember that making lame statements such as "It's different when men love women" in an attempt to justify why you think Edward's (presumed) adultery with men is nasty and icky while his grandson John of Gaunt's adultery with Katherine Swynford is fabulously romantic, looks bigoted. There are ways that we can discuss prejudices of other eras without making it look as though we share them and expect our readers to do so too.
EDITED TO ADD: My friend at the wonderful A Nevill Feast blog has written a great post about women in historical fiction. Please read it!