Even if Snow was a horrible child, and we have seen no evidence at all that that is he case, that is not an excuse for what Regina did. The only reason is that she snapped when Daniel was killed. And snapped again when she learned that Snow told Cora when she asked her not because it was such a simple thing to do, don’t tell Cora, and Daniel would be alive.
Snow is a child and did not understand what she was doing, but i think that Regina’s reaction still feels real to me. Especially if Regina believes that Snow did it on purpose so that Snow could have Regina for herself and as her mother. Meaning it seems like Regina believes that Snow told Cora on purpose knowing that Cora would get between Daniel and Regina and that Snow could then have Regina as her mother. I am not saying that is true, but it seems that Regina believes that. That Snow did it on purpose.