Why is Killer Bee still alive?
According to this link


  Madara then proceeded to chain them up and prepared to remove
  Yang-Kurama and Gyūki from their jinchūriki. Struggling against its
  confines, Gyūki apologised to B as it was extracted and sealed within
   the statue along with the other tailed beasts, 
  but not before cutting
   off one of its tentacles, letting B survive through the chakra within
   it.


So in short he was having chakra worth of one tentacle which was cut off from Gyūki and just because of this he was able to survive.