trial. the alleged killer is charged with first-degree hurd in the death of a new hampshire -- murder in the death cough a new hampshire college student. the star witness is his former girlfriend, and she's on the stand again today. patti ann brown from our new york city newsroom with more. >> reporter: it's a by sawr case, accused of raping and strangling a 19-year-old woman, elizabeth marriott, of massachusetts, was a student at the university of new hampshire when the killing took place in 2012. the star witness for the prosecution be, as you mentioned, is seth's former girlfriend, katherine mcdonough. she originally told investigators she was engaged in rough sex with the female victim when she died. but after being offered immunity from prosecution, mcdonough changed her story saying seth killed lizzie after she rebuffed his advances. you are looking live at the trial right now. today she insisted she is not into violent sex. >> you wanted to get away from any notion that you would have had sexually dominated liz