Ruthlessness is when someone doesn't have any kindness or pity and does whatever they want, no matter who it hurts.
Ruthlessness is usually seen as a bad thing because it means someone is not thinking about other people's feelings.
Examples:
His ruthlessness in business made him very successful.
The ruler's ruthlessness scared the people.
She showed her ruthlessness by not helping the hurt animal.
The cat's ruthlessness in catching mice is impressive.
The dragon's ruthlessness in guarding the treasure was legendary.
The pirate captain's ruthlessness was known throughout the seven seas.