Sometimes the lines are blurred when it comes to someone being ill and them acting terribly. It can be exploitative because, at the core, this person may feel intensely terrified and in agony emotionally. So one of the few ways that they've learnt to cope with that pain and to lessen it is by resorting to harsh and "terrible" behaviors to feel secure.

It's not an easy situation to be in but it is complex. Altho if the situation isn't balanced out and you aren't getting a leveled amount of enjoyment and lets say benefit out of the relationship and it is affecting you negatively then maybe you should not be a part of it as it is not your responsibility to be there and to take the punches.. and maybe you're better off without that person being around you.