die Grausamkeit
cruelty
The quality of deliberately causing pain or suffering to others without compassion.
Die Grausamkeit des Krieges ist unvorstellbar.
The cruelty of war is unimaginable.

cruelty
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cruelty
The quality of deliberately causing pain or suffering to others without compassion.
Die Grausamkeit des Krieges ist unvorstellbar.
The cruelty of war is unimaginable.

'Die Grausamkeit' (cruelty) is derived from the adjective 'grausam' (cruel). The ending '-keit' is often used to form abstract nouns from adjectives that describe a quality.
Sein Verhalten war grausam und unverzeihlich.
His behavior was cruel and unforgivable.
The meaning depends on whether the word is used countably or uncountably. In the plural ('Grausamkeiten', cruel acts) or with an indefinite article ('eine Grausamkeit', a cruel act), it always refers to concrete acts. Without an article and in the singular, it describes the abstract quality.
Die Grausamkeit mancher Menschen ist schockierend.
The cruelty of some people is shocking.
When 'Grausamkeit' (cruel act) refers to a concrete act, the verb 'begehen' (to commit) is often used.
Er wurde beschuldigt, unzählige Grausamkeiten begangen zu haben.
He was accused of having committed countless cruel acts.
humanity
'Menschlichkeit' and 'Grausamkeit' are clear opposites. 'Menschlichkeit' means compassion, kindness, and respect for other people. 'Grausamkeit', by contrast, means behavior in which someone deliberately causes suffering and shows no pity. You can also use 'Grausamkeit' for a single terrible act. If you want to show that someone acts humanely and compassionately, 'Menschlichkeit' fits. If someone acts harshly, hurtfully, or without compassion, 'Grausamkeit' fits.

Die Grausamkeit des Täters erschütterte die ganze Stadt.
The cruelty of the perpetrator shook the whole city.
Die Ärztin behandelte den alten Mann mit viel Menschlichkeit.
The doctor treated the old man with great humanity.