Cruelty is a hidden trait that reflects how many capricious or violent actions Elodie has taken. It can be raised by certain choices and activities, and it both locks and unlocks certain outcomes. Cruelty checks are usually invisible to the user, but certain events help you determine how cruel Elodie is.
Increasing Cruelty is mostly predictable: be violent, be capricious, order lots of executions. Also predictably, not many will like you, so be prepared to deal with angry nobles, assassins, civil wars and revolutions.
For the cruel player, one skill that you may need is high Wield Magic. It's used in two cruelty-increasing choices, and can help with other obstacles common in the cruel story path such as the naval battle, the civil war, the magic ritual and the peasant revolution. It's possible for a non-Lumen Elodie to finish a cruel story path, but it's considerably more difficult given the different skills needed.
Cruelty >5 is a point where most cruel actions are unlocked, and Elodie is considered evil, though not unredeemable. Cruelty >10 unlocks the most despicable actions in the game and is considered a "point of no return" for Elodie.
Corisande may visit, asking for Briony. If you sent her off into the Old Forest alone, couldn't talk her out of it or went with her but abandoned her to her fate, Corisande will accuse you. Choose "It was your fault".
If you won the naval battle without the magic ritual, you will be holding prisoners. Either choose "Ransom the rich, execute the poor" or "Execute them all"
As noted, there are almost no ways to decrease cruelty, and saving Briony while in the Old Forest is one of the only ways. (Need to check if sending her home in the case of civil war during Week 29 reduces Cruelty).
Be aware that high cruelty values will sometimes override the outcomes of certain events. Case in point: The Visit Dungeons weekend activity, the amount of enemy losses during the civil war, reactions to the poisoned chocolates, and many epilogues and romance options.
If you pass any check that unlocks "Save (the chocolates) for later", pass an Intrigue check to unlock "Test for poison". If you fail the subsequent Medicine check, pass a Cruelty check to have Elodie force-feed the poison to a chicken (near-zero Animal Handling) or to a prisoner.
If you lose the naval battle, you will meet Togami. If you had a conversation with you magic tutor about spare Lumen crystals, pass a Cruelty check to succesfully "Offer to give him your power". This ends the game immediately and unlocks both the "Evil Elodie" epilogue and the "Become an evil minion" achievement.
If you succesfully performed a magic ritual, you will be informed of the Kraken it unleashed. If you known Charlotte has magic powers, pass a cruelty check to unlock the choice "Sacrifice Charlotte". Be wary that if you sacrifice Charlotte and Lucille is alive, she will kill you next week without any chance of checks saving you. Also there are other ways of unlocking said choice.
As mentioned before, there are certain events that help determine your cruelty values before the endgame or the epilogues, some mentioned above. These are:
Reiceiving either the bouquet of flowers, the dancing figurine or the embroidered cushion. If you choose "Give it to Alice", a success will confirm >1 Cruelty, a failure confirms <1 Cruelty (that's to say, no Cruelty).
If the outcome of the Visit Dungeons weekend activity is changed to increase Cruelty +1, Cheerful +1 and Angry +1, it confirms >5 Cruelty.
If Elodie saved the chocolates for later, when she's informed a steward died from eating one, if the reaction is Cheerful +1, it confirms >5 Cruelty (though, at this point it's too late to change it).
If Elodie passes a Battlefield Medicine+Herbs check during the civil war, flavor text indicating her saving the lives of rebel soldiers confirms <5 Cruelty.
If Elodie passes a Cruelty check during the civil war (her soldiers follow her example and massacre more rebels), it confirms >10 Cruelty (too late to change it and enough to unlock the two most evil acts in the game).
Like above, ransoming Briony and succeeding at the option "Exile for the rebels" can confirm >10 Cruelty, but it's also affected by Presence.