Why does George Wilson lock Myrtle in the bedroom?

The answer to this can be found in Chapter 7.  Basically,
he thinks that she is having an affiar.  So he locks her in the bedroom.  He tells his
neighbor that he is going to keep her there for a couple days.  After that, the two of
them are going to leave and go live somewhere else.  This will keep her from resuming
her affair.

Of course, it is Tom Buchanan that has been
having an affair with Myrtle Wilson.

Later on, Myrtle slips
out past George and out of the house.  That is when Daisy runs her over with the
car.