If you are comfortable with your choice, then you can just say that you have not maintained contact and do not wish to because the relationships were not good for you.
Why do you question yourself? What about not contacting your family would make you "bad"? I think, when you say, essentially, that they could be dead for all you care, it is the anger that people feel and respond to....maybe you need to feel sad that you don't have family you Want to contact...
Is it the anger you can't let go of? Or the pain? Is your life better now, without them? Even when you leave a family, it is hard not to carry them with you in some way...