NTA You needed to end this arrangement immediately because her child clearly has behavioral issues, and it sounds like you're lucky your child had not been injured yet. Chances are she's just trying to guilt trip you over the arrangement coming to an end because she knows it's going to be nearly impossible to find someone else, especially a professional, willing to care for her child. And with that child's behavior issues, no daycare would accept them either, which there's a strong chance she's already aware of.

This so-called friend most likely took advantage of your "friendship" because they were desperate for childcare for their problematic kid. Please just cut your losses and don't try to maintain this friendship. And whatever you do, do NOT let her guilt you into ever watching that child again.