What is the moral lesson George has learned from the one time he ordered Lennie to jump?Of Mice and Men by John Steinbeck

In my opinion, what George learned is that it is wrong to
bully weaker people.  Put differently, he learned that when you have more power than
someone else, it is not morally right to use that power for your own amusement. 
Instead, you have to consider what is best for those who are weaker than
you.

George says that he used to enjoy pushing Lennie
around.  He got a kick out of using Lennie to make himself feel smart and powerful.  But
then he ordered Lennie to jump in the water, Lennie did and almost
drowned.

At that point, George learned that people with
power over others must not abuse this power (this is something that Curley needs to
learn but has not yet learned).  George is not yet past being mean to Lennie at times,
but he has certainly learned to rein it in more than he used to be able
to.