expression, suru verb (irregular)
harden one's heart; steel oneself
Used when one must suppress compassion or hesitation to do something difficult but necessary, such as scolding a child or firing an employee.
泣いている子供を置いて出かけるとき、心を鬼にした。
I hardened my heart when I left my crying child and went out.
上司は心を鬼にして、彼を解雇した。
The boss steeled himself and fired him.