Why is it ironic that Lennie is shot with Carlson's Luger in Of Mice and Men?

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.