Even though Mariam doesn't like Laila at first in A Thousand Splendid Suns, why does Laila defend Mariam from a beating from Rasheed?

Laila defends Mariam for several reasons.  First, she feels pity for Mariam.  As horrible as Laila's situation is (losing her family and her lover), she realizes that Mariam's life with Rasheed has not been any better.  Secondly, Laila knows that she needs an ally.  Although Mariam treats her harshly, Laila must be able to form an alliance with Mariam simply for survival's sake, if nothing else.  Finally, Laila--unlike Mariam--grew up in a loving family with a father who was kind and gentle.  She instantly recognizes that Rasheed's treatment of Mariam is not acceptable, whereas Mariam is used to not be treated well.