Indiscriminate killing of large numbers (8)
I believe the answer is:
massacre
'indiscriminate killing of large numbers' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for massacre that I've seen before include "Brutal slaughter" , "Butchery" , "Murder (of many persons)" , "Slaughter, pogrom" , "Widespread killing" .)