Final answer:
Slim is the character who understood why George had to shoot Lennie, recognizing it as an act of mercy to spare him from a lynching led by Curley.
Step-by-step explanation:
The individual who understood why George had to shoot Lennie in the novel Of Mice and Men was Slim. Unlike the others, Slim recognized the deep bond between George and Lennie, and he understood that George shot Lennie as an act of mercy, to spare him from a worse fate at the hands of the angry mob led by Curley. Slim's quiet nod to George served as an acknowledgment of the difficult and compassionate choice George had made.