I always, ways, ALWAYS advocate for keeping to only one ATF per tank due to how easily they accidentally maim each other in regular interactions, and that's ignoring the outright aggression you seem to have experienced.
Batf and to a lesser degree fatf are known to occasionally get a violent hair up their butt. Your fish is a fatf. If by some means you ended up mixing a fatf with other non-fatf atfs, then the fatf would kill them. I once had a 4" fatf that harassed a then 10" gatf so badly I had to separate them.
Long and short of it is that I think you ended up with a fatf mixed into a group of something else, and the fatf did what fatf do.