Do you think it's ok to a child for reasonable cirstances?

My cousin who is 8, who's parents don't want to discipline, threw a toy truck at his brother and almost broke his tooth. I had already said that they were not to interact with each other for the rest of the day. That was just blatant disrespect. I ed him on the and he told his mother. She scolded me in return and let him have ice cream while watching some violent movie. I don't think that what I did was wrong, especially when I had stated earlier that I wouldn't tolerate this kind of behavior. I think that this way of "disciplining" is write and I would love some feedback on this problem of bratty kids and useless parents.