aggressive behaviour is not defensive behaviour in every single case as there are differences in behaviour between balls that clearly have a tendency for one or the other or a combo of both, misnomers or not.
i see what you are saying and trying to do but i feel that there is also a perceived degree of aggression/defensiveness determined by the keeper and that some may not mind working with the animals that you may find aggressive/defensive.
i think its interesting though that you are working with the behavioural aspect of these animals and im not trying to argue that but feel that culling these animals may not be necessary.