I got the impression that she really didn't "die" until 1840. Throughout the series, she seemed to be an immortal, so no matter what happened, she would always me around in some form or another, always ready to antagonize Barnabas.
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....she was shot this time in the shoulder, she was able to mend herself eventually after she "came to". She wasn't able to do the same for herself in 1840, when she was shot in the heart by Lamar. She had givnen up her powers for Barnabas by then.....(please refresh my memory if I recall this wrong).
Karen #2