Guenevere was such a bitch for cheating on Arthur with Lancelot, wasn't she? And she and Lancelot deserved to have that Excalibur sword thrusted into the ground between them by Arthur for betraying him, didn't they?
shareNo
shareLancelot was in a bad place. He was in love with his best friend's wife and knew he shouldn't be feeling that way, but he did. It's hard loving someone you know you can't have.
shareThat is hard. It's also hard to find out that your best friend and your wife are having an affair behind your back while you were dragging a country out of chaos and war.
The best versions of the Arthurian legends are ones where the love triangle is balanced so that nobody seems completely at fault and where it is a difficult decision for Guenivere to decide between Arthur and Lancelot. When it's just, "Oh, those poor star-crossed lovers!" it's just not as interesting a story.