The Value of Pain and Sacrifice

The law also implies that a lesson without pain is meaningless because no one can gain anything without sacrificing it.

In the character arc of Roy Mustang, we witness that he suffered and felt the pain.

Roy Mustang

He learned the painful lesson of the cost of power and the use of alchemy as a weapon that caused complete destruction and suffering during the Ishvalan Civil War and the pain of losing his best friend. 

He was motivated to overhaul the military system and rebuild the country after the incident, which is a sign of humanity in his character development while suffering great pain.