Mr. Berry is Bee's father and abuser.

When Zayne found Bee during that raid, Zayne was filled with rage like they had never felt before, and sought to unleash what he did to Bee upon him tenfold.

They hunted Mr. Berry down and hung him from his feet from a streetlight in the middle of the night, and cut at him, before setting him on fire and dismembering him, leaving him completely unrecognizable.

Zayne was horrified at their self because, not only was this excessively grotesque (but, arguably, kind of deserved), this was their first time feeling rage, as their telepathic abilities had kept them numb and void of any emotion until now.