let me consult my crystal ball. hmmmm, i cannot see clearly. in reality, who could possibly tell you who's the culprit. it could be any of the fish you've got, and the tails might have gotten bitten after they died rather than before?
its most likely the gouramis. they can be nippy and they sometimes chase the other inhabitants of the tank (especially males) if they are smaller then they are
see .. i knew that would happen. no matter which fish anyone picked as the culprit, you'd suggest that 'no, it couldn't be him'. it's basically a guess so why put any stock in any answer.