I understand that he would have been married to Cora before they both attained power. But after Cora got her power and position, I don’t see how she would have left any loose ends. She said sacrificed a lot to get where she was at. There does not seem to be anything she wouldn’t have done to keep everything that she worked for. Also, I don’t see her being happy if he ex-husband got power, especially if it could pose as a threat to her, I don’t see her letting it slide–he might have wanted to cause trouble just to spite her, or at least that is what she would think.
Regina was already pretty twisted–Daniel’s death made her hate a ten year old girl. So i doubt killing her mother (if Regina did) would increase her hatred by much.