Why does George shoot Lennie? Do you think this was the right thing for George to do

When reading Of Mice and Men, it's
easy to identify with what George does and feel relief that he's "put Lennie out of his
misery." 

It's a bit different, though, when one looks
objectively, instead of subjectively, at the situation.  When one steps back and gets
some figurative distance from the situation, it's not so easy to side with what George
does.

Lennie is not an old, sick dog.  The parallel between
Lennie and Candy's dog actually reflects negatively on George's mercy killing.  Lennie
is a human being. 

One has to ask, at what point should
George not kill Lennie?  How much more mentally able does Lennie need to be, before it
becomes wrong for George to do what he does?  And who makes that
decision? 

A novel is easy.  We talk about it and know it's
not real.  But as far as the ethical dilemma itself, the issue isn't so
simple.