I always had more the feeling that Kara's scream at that moment had less to do with Sharon personally then the fact that Kara had reached a breaking point.

Between the physical beating and the emotional ones she'd endured leading up to this point, it seemed the last straw. And it struck me that it was Helo she was predominantly reacting to. Here he was alive when she'd thought him long dead, and not only was he protecting a Cylon but claiming that she was pregnant with him as the father to be.

The fact that it was Sharon, someone she had trained and teased--and left back with an unsuspecting fleet--just tipped her over the edge, IMO.