Had I been in the place of your mom, I think I would have been concerned too, both because of the possibility that you might hurt yourself and because I know I'd carry immense responsibility on my shoulders if you hurt yourself or anyone else.
Saying that the only thing that prevents you from doing
anything is the possibility of being caught seems to imply a lack of an internal deterrent that doesn't rely on fear of being punished. It appears to me from what you told her that you were trying to make that clear to her, which would have definitely worried me if I had been in her place, even though I'd most likely have assumed you were misrepresenting how you would actually act in the case of being safe from "getting caught."
I think that "good," "bad," "right," and "wrong" are all relative terms that can and mostly do vary from person to person, society to society, and from one religion to another, bar some bare "basics" that appear to be shared among most people regardless of where they are or what they believe, such as "don't murder" and "don't steal," etc.
However, in a situation like the one you described in your OP, I wouldn't really care about the differences in the way we use such labels as long as they don't hint at committing future crimes or hurting other people; I'd care most about the actual consequences of what the use of such labels would entail.
For example, if someone told me that they think murder is a "good" thing, I'd be concerned about the harms that such a view could entail. It would be far more than a simple difference in our worldviews expressing opinions that don't uphold not causing harm to others can be a good indicator of future misbehavior. I don't think it comes as any real surprise that someone may consider abstract concepts to be very relevant to the actual world when such concepts can and do influence how many people act.
Having said the above, I think it's worth pointing out that I'm not implying that what you told your mom necessarily means that you'll engage in illegal conduct in the future... since I think that what you said may well be misrepresenting the way you'd act in real situations that lack an external punishment as a consequence of committing a crime(s).
Also, have you considered that your mom's shock and "flipping right out"
might have just been out of concern for your own well-being rather than any difference in ethical or philosophical approaches?