Karen Nix is the stalker of Miranda Shea and the murderer of her husband Larry, which Miranda manipulated her to do by starting an affair. Karen met Miranda in rehab, and she had a restraining order on her for harassing another woman.
Karen was devastated to find out Miranda strung her along to use her, so she testified to Miranda pleading Karen to kill Larry out of a lie of abuse. Karen took a plea of a reduced sentence for her testimony, and Miranda was eventually convicted. (L&O: “Obsession”)