In Of Mice and Men, is George's final act toward Lennie justified?

Absolutely.  Assuming that you are talking about George
killing Lennie, what he did was absolutely justified and might even have been the only
moral thing he could have done.

George knows that what
Lennie did is not really Lennie's fault.  Lennie is just not bright enough to truly be
guilty of murder.  But Curley is never going to see it that way.  He has sworn that he's
going to hurt Lennie before Lennie dies and there is no reason to doubt that he will
since his father is a powerful man in the area.

So George
has two options -- kill Lennie himself or let Curley get his hands on Lennie.  Given
that choice, killing Lennie in a merciful way is much more humane and is therefore
justified.