Tetras are egg scatterers, more likely to eat their own eggs but never guard them. They're not usually territorial either, may be something wrong with that one. One of my 5 Colombian tetras was hanging out alone at one end of the tank for a few days before I realized it was missing an eye. Few days later it was completely missing, so I picked up 5 more to increase the size of the group. All 9 seem to be doing fine, no clue what happened to the 1 that was injured.