In the novel Of Mice and Men, by John Steinbeck, there are two instances that portray a killing done for the purpose of mercy. The first of such deaths was that of Candy's old dog. Seeing how the dog isliving a miserable existence, Candy chooses to put the dog down. It is Carlson who doesit, using his own Luger.
This is a foreshadowing of what is to come next: Lennie's shooting would be the second mercy killing in the story and, ironically, by the same weapon. This time it is George who makes the final decision of killing Lennie for the sake of not allowing Curley's mob to torture and kill him as a revenge for Curley's wife's accidental death at Lennie's hands.
Carlson is, also ironically, one of those who would have been in the mob. However, he could not find his Luger. This is because George had stolen it when he was watching the body and used the weapon to shoot Lennie at close range, killing him instantly (we assume also painlessly), and sparing him from the monstrosity that was coming Lennie's way.
This is the irony of the killing of Lennie with Carlson's Luger.
It is somewhat odd that the gun owned by Carlson should be a Luger. This is a German automatic pistol with a distinctive shape. It looks entirely different from an American revolver or a U.S. Army automatic. Steinbeck makes a great deal of the fact that the gun is a Luger. He describes how Carlson removes the clip and cleans and oils the handgun after shooting Candy's dog with it. It is not implausible that Carlson should own a German Luger. Conceivably he could have picked it up as a souvenir during World War I, or he could have bought it from some veteran of that war. Steinbeck's whole purpose in making the weapon a German Luger was to make the gun easily identifiable when George takes it out of his coat pocket at the end of the story and uses it to kill Lennie. We understand that it was George and not Lennie who stole the pistol from under Carlson's bunk because we recognize the gun by its shape. We also understand that George knows how to use it, because he was watching when Carlson was cleaning it after using it to kill Candy's dog. Here is just part of Steinbeck's description of Carlson's handling of his gun:
Carlson found a little cleaning rod in the bag and a can of oil. He laid them on his bed and then brought out the pistol, took out the magazine and snapped the loaded shell from the chamber. Then he fell to cleaning the barrel with the little rod.
The whole business of Carlson shooting Candy's dog seems to have been invented to establish that the Luger existed, where it was kept, and how it worked. This makes it plausible that George would be able to obtain a gun when he decided to kill Lennie, that the reader (or theater audience when the book was adapted into a stage play) would immediately recognize it as Carlson's Luger, and that George would understand how to load and fire this foreign weapon.