I think that it is quite easy to find issues of cruelty in the drama. Steinbeck constructs is so that these are evident on personal and social levels. I think that there is a social level of cruelty in how jobs are scarce and these men have to go from place to place in search of work. Little in way of help or assistance is present to these workers, these "bindle stiffs" whose existence is defined in a migratory and transitory manner. At the same time, the manner in which these men interact with one another is one where there is little room for compassion and care, but rather one in which harshness defines their state of being, one in which individuals are afraid of losing their own share at the hands of someone else.
In terms of how the characters relate to one another, the killing of Curley's dog at Carlson's hands probably represents one element of cruelty. Adding to this is how Steinbeck constructs it with the presence of "the silence" in the period between Carlson luring the dog outside and the waiting for the shots. Naturally, Curley is one who uses cruelty as a wedge to interact with the other men, if only to masquerade his own insecurity. I think that the reality that men like Crooks face could be considered another form of social cruelty that is reemphasized on a personal level. When Curley's wife interacts with both Crooks and Candy, it is a moment where stinging cruelty and venom is apparent in terms of how she has power and they lack it. It is here where cruelty is evident in the narrative.