Rukia's speedy execution was ordered by the Chamber of 46, whomat that pointhad already been murdered.  All those orders came from Aizen for the purpose of recovering the Hogyoku.  (The method he ultimately uses was a backup plan he found in Urahara's research records while pretending to be dead.) 

 :As for the matter of ganging up on Arrancar...that's just stupid.  This is a fictional story, after all.  It wouldn't be very interesting if the bad guys got killed off so easily.