This achievement requires more than a bit of dark humor. If you don’t mind playing as a heartless tyrant, you should be able to earn this quite naturally. However, if you’d rather not toy with the lives of others but you just really, really want to complete all of the achievements…

The easiest way to do this is to avoid marrying anyone in particular until you’ve executed your first lord or lady. You can then check the Encyclopedia in the campaign (by pressing N on the keyboard) to search for the lord or lady that you’ve slain, and see who their partner was. Now you can seek them out and work on winning their affection before proposing marriage.