Because he is old and useless...he only has 1 good hand..etc
In the novel "Of Mice and Men," Candy is missing a hand, not a limb. After losing his hand in an accident, he becomes self-conscious and feels marginalized on the ranch where he works.
Opinions on Candy in "Of Mice and Men" vary, but overall he is seen as a symbol of loneliness and isolation, as well as a representation of the vulnerable and marginalized in society. Some may sympathize with Candy's situation and see him as a tragic character whose attachment to his dog mirrors his own fear of being discarded and forgotten.
Candy
because they killed his dog
In "Of Mice and Men," Candy receives $250 as compensation for losing his hand in an accident on the ranch.
Candy tells George that he should have killed his dog on page 44 of Of Mice and Men.
because they killed his dog
Hope for Lennie and George.
They are both old
because they killed his dog
Candy asked for some painkiller to help with his stomach ache in "Of Mice and Men."
Candy tells the story of losing his hand on page 61 of "Of Mice and Men" by John Steinbeck.