The thing that really strikes me about this is it's not anywhere near the realms of normal office bitching. I'd imagine all of us have a whine about people who don't pull their weight, spend all their time on the phone to their DP, make messes you have to clear up, etc. That's normal, some people are bound to rub each other up the wrong way. I have one colleague who has a tendency to make unpleasant remarks about others' appearance, no-one pulls her up on it because we're all nervous about getting on her bad side but uncomfortable silence descends when she does this, it's not pleasant at all, but still within what I'd consider normal workplace behaviour (just).
Expressing a desire to smash someone's face in and wishing they'd kill themselves is just totally freakish behaviour. If I overheard someone talking this way, even if it was about someone I didn't like, I'd be shocked. To hear it about yourself must be beyond awful. Big hugs for you, you sound thoughtful, decent and kind.
Please report what she said though, she could do this to someone else and, if she still has an unblemished record at that time, they might not be believed. She is unlikely to get into serious trouble when it is just one incident and one person's word against another, it will stay on her record though. Hopefully that will be enough to prevent her ever doing this again, but if she did, the person she did it to would instantly be believed.
Also you do need to think about covering your own back. I had a situation with a colleague (who had been a friend before we even started working together - which made it much worse). There was an incident at work where she did something underhand, which nearly resulted in me losing holiday I'd booked, I chose to forgive, forget and move on rather than making an official complaint. She then did something unforgivable unrelated to work, I stopped speaking to her apart from the bare minimum required to be professional. She made a complaint about me and I was hauled into the office for investigation. I've never been in trouble at work before or since, no action was taken against me (because there was no basis for her complaint) but I could never forget it was on my record until the day I left (for a much better job - hah!).
Sorry this is so long but I wanted to share my experience, workplace bitches often try to get their victims into trouble. I wish I'd spent less time giving second chances, because of a friendship I'd thought existed, and made some formal complaint about the first incident.