Sad Essences can be obtained by being mean to other Sims in MySims Kingdom. There are many minor ways to obtain many of these essences, but being mean to other Sims is probably the most efficient way to collect many of these essences. However, being mean will not deduct any of the Sim's relationship, so it should be safe. Unlike in the original MySims, all Sims will drop sad essences when you are nice to them, no matter who they are. However, you are not able to do be mean interactions with Buddy or Lyndsay, so you must try being mean to other Sims.