[@Pathfinder]

It was hurt and upset (hence the screaming for glitching out before the body actually appeared), probably recognizing Garnet. Cluster is a forced fuse of broken gems. Broken gems can't make bodies but when they were mashed like that they could make up a body that was well creepy. It is an unnatural thing to do which is why Garnet started falling a part. It was an insult to the memory of her comrades and also a jab at the crystal gems, revenge in a way for the rebellion.