its unlikely that it was killed deliberately. if indeed it was injured, it could have been taken care of by the zebras and the empress.
and if it died naturally, then the gang couldn't have passed on a great feast.
sorry for the loss. but please don't be upset too much. this is a way of a tank balancing itself out. even after cautious stocking, very rarely do all malawi'ans put in as juvies survive to adulthood. these are rough fish, so please be prepared for casualties.
just to drive home the point, yellow labs are usually touted as the mildest of the mbuna bunch. this weekend i was at my lfs
. they have a 120g tank, ruled by adult oscars. someone had brought in a 5" yellow lab and someone else, two tilapia buttikoferis. i stared at the tank in amazement as i saw the new lab rule the roost with absolutely no trouble whatsoever. if you have kept buttis, you'd know how mean they can get. but, the point is, the lab managed to viciously bite, head-butt, and chase each of these bigger fish from its territory.