This is the 8915th most frequent Japanese word.
残酷
Cruelty; brutality.
Here, '残酷な' is used as an adjective describing '運命' (fate), emphasizing its harshness and inescapable nature.
残酷な運命が彼を待っていた。
A cruel fate awaited him.
In this context, '残酷さ' is a noun derived from '残酷', highlighting the characteristic of cruelty exhibited by people.
その映画は人々の残酷さを描いている。
That movie portrays the cruelty of people.
'残酷に' is used as an adverb modifying the verb '感じられる' (feel), demonstrating the manner in which his words impact others emotionally.