noun
Refers to a cruel, brutal nature or disposition; often used to describe acts or character.
彼の残忍性には驚かされた。
I was shocked by his brutality.
その事件の残忍性が問題に
The cruelty of the incident became an issue.
残酷 is a more common word for 'cruelty' or 'cruel', often used in everyday contexts; 残忍性 is more formal and emphasizes the inherent brutal nature.
Compound of 残忍 (ざんにん, 'cruel, brutal') + 性 (せい, '-ness, nature').