What is the theme relating to vulnerability or empathy in the story Mice and Men?

Many of the characters in the story are vulnerable. Candy is vulnerable because he's an old man, who is seen as useless. Lennie is vulnerable because of his innocence, Crook's is vulnerable because of his position, and Curley's wife is vulnerable because she seeks companionship.

In terms of empathy, I'm not seeing much empathy in the characters themselves. George's act of killing Lennie might be seen as empathetic. Slim's assistance might possibly induced because he feels empathy for George and his position. Empathy is primarily generated in the reader, as the events unfold.