Why didn't Kira kill manipulate the mafia to kill Mello?
There is a rule indicating that someone's death must not cause another's:


  Whether the cause of the individual’s death is either a suicide or
  accident. If the death leads to the death of more than the intended,
  the person will simply die of a heart attack. This is to ensure that
  other lives are not influenced.


From http://deathnote.wikia.com/wiki/Rules_of_the_Death_Note