I expect to gather quite a few downvotes for this viewpoint but here it goes. There is no way I would have intervened. I am a male and I can't see my involvement in the situation working out for me at all. Now you might say I'm wrong for putting my self-preservation above that on Kristen's but this isn't just about preventing physical harm to myself. In fact if I knew that I would only be cut (non-fatal) then I would…
"suddenly we were brawling, she swinging at me with a knife in one hand"
"I felt blood flowing down my face. 'SOMEBODY HELP ME!' People stare and watch."
"Please, she’s got a knife, she’s trying to stab me, please! ANYONE!"
Seems to me that it was pretty clear that actual knife attacking was taking place. She's got blood streaming down her face crying blue murder about a girl with a knife - you wouldn't help because she hadn't actually been stabbed yet?
How would you live with yourself?