Guess it is because a red dragon was already almost dead (lost both wings) and had 0% chance to survive so she would tell some trash before passing away. But damn Rurin did overkill her.
You don't seem to understand.
The red dragon learnt that there's a dragon in her cellars, went down to meet her, found out it's a mighty black dragon, and started provoking her knowing she'd probably end up dead. Why the fuck did she start to heckle her?
Well, because the author wanted a way to shoe in some details on her mother.