Every story must have its heroes and its villains. A story with only good guys would be quite a yawner (and a world with only good girls would be a sorry place indeed).
I’ve not treated Guinevere kindly, I’m afraid. That didn’t exactly happen by design. When I was researching Morgan le Fay for Strange Bedfellows, I came across the following fact from an article on the literary tradition of Morgan.
Her enmity towards Guinevere has its origin in the Vulgate Lancelot, where Morgan is having an affair with Guiomar, Guinevere’s cousin, and Guinevere puts an end to it.