why did Carlson wanted to mercy kill Candy's dog

In part three of Of Mice and Men Carlson wanted to mercy kill Candy's dog. Before Carlson offered  to kill Candy's dog he said to Candy, "God awmighty, that stinks. Get him outta here, Candy! I don't know nothing that stinks as bad as an old dog. You gotta get him out"(44). After awhile of Carlson and  Candy softly arguing about the dog Carlson offers to shoot the dog for Candy and Candy agrees. I think Carlson did this was because he felt sorry for Candy and thought that it would be better that Candy doesn't see it happen. I also think because Candy won't shoot the dog so that it won't have to be in suffering the older the dog get. 



     Why do you think that Carlson wanted to mercy kill Candy's dog