'I think the butler did it.....' We have no idea which fish is agressive.

when any of the fish usually die almost every living fish give some nibble to dead fish's fins and tails, nipping might not be the cause of death, i mean to say it is most probably water quality issue may along with fin rot. Regular 30%water changes may help both issues use some salt or malachite along and see. Guppies play chasing each other, watch them a moment and you will know the extent of aggression in chase. Good luck to you and your fish.
 
Welcome to AC!
 
AquariaCentral.com