Could it be interpreted that George kills Lennie in Of Mice and Men because he is sick of standing up for him?

In Steinbeck's novella, Of Mice and
Men, there are clearly some suggestive parallels between Candy's old dog and
Lennie.  The old dog is no longer of any use and it is of diminished capacity just as
Lennie is by the narrative's end.  Carlson tells
Candy,

"If you
want me to, I'll put the old devil out of his misery riht now and get it over with. 
Ain't nothing left for
him." 

Similarly, George puts
Lennie out of his future misery and "gets it over with." George knows that there is
no reversal of fortune for Lennie anymore than there has been for Candy's old dog.  For,
as he and Candy stand over the body of Curley's wife, Candy
says,

"We
oughtta le 'im get away.  You do't know that Curley.  Curley gon'ta wanta get 'im
lynched.  Curley'll get 'im killed."

George watched Candy's
lips. "Yeah," he said at last, "that's right, Curley will. And' the other guys
will."

The true friend of
Lennie, George cannot bear to think of the cruel fate that awaits Lennie, one that will
include great torture since Lennie cannot get away, nor can he survive on his own, and
George can no longer protect him.  Their dream is destroyed; George has said, "I think I
knowed we'd never do her" and he is in despair.  He and Lennie have no future; they
cannot run from Weed and Salinas and who knows where else.  Lennie, tragically, has
outlived his usefulness, just as the old dog has, and there is no place for him in the
world; "there ain't nothing left for him."  Therefore, George shoots his friend as a
mercy killing. Afterwards, Slim tells him, "You hadda, George.  I swear, you hadda," As
Steinbeck himself has written,

readability="6">

"Lennie was not to represent insanity at all but
the inarticulate and powerful yearning of all
men."

Lennie's death, then,
symbolizes the futility of this yearning for meaning and manhood in the alienated world
of the dispossessed during the Great Depression.