Before he died, Juan's father gave him one piece of good advice. Actually, he probably gave him lots of pieces of good advice, because he sure used to give out a lot of advice, but Juan has forgotten most of it and failed to put most of the rest into practice, so he's not sure which parts were good. But! One thing he did say, and which Juan did take to heart, is that you should never have sex with a woman because she wants you to. Thinking about whether women want to have sex with you leads to poor decisions, like sneaking into the house of a rival nobleman to fuck his wife, who has been absolutely throwing herself at you, only to get caught in the act. Then the husband kills the wife, which is a bit disappointing, because she was gorgeous, and more importantly is extremely angry about your behavior, which loses your father... some kind of important political thing, he doesn't remember except that his father was furious. Anyway, you should not have sex with women because they want to have sex with you. You should, instead, have sex with women whose husbands can't do shit about it. This is actually, it turns out, also pretty great. There's nothing like dictating terms to a rival you've defeated, and throwing in "also, I'm going to fuck your wife", because you can.

Anyway, when the convention is cancelled for multiple days, Juan gets bored. Not that the convention wasn't already kind of boring, but now he doesn't even have anything he's supposed to be doing. He does try a brothel, one evening, but they throw him out, and after that it's raining again. So he walks along a line of ragged houses, the kind nobody who matters would be caught dead in. He turns on the charm, asking for shelter from the rain. It doesn't take all that long, really, to find a woman who's interested enough to invite him inside, and it isn't all that many more conversational steps from there to the bed.

Unfortunately, the husband has some really shit timing, and comes back before Juan is done. Fortunately, Juan has taken his father's advice, so this is a mere moment's inconvenience. Juan casually cuts the man's head off, and then goes back to what he was doing. The woman is screaming and sobbing, now, but that doesn't really change anything. A dead husband can't do shit.

He doesn't kill her, after. He's a nice guy like that. He doesn't hide the body, either, because it doesn't at any point occur to him that anyone who matters might care.