Should one see another STABBING someone, one must not run away. NO mortal has the right to harm another with a weapon such as a knife, regardless the aggressor or victim is in the right or wrong. Only the Court has the power to decide.
Throwing items at the aggressor is the right thing to do, while another calls 999 & ambulance for help, to delay further harm to the victim. Throwing items at the aggressor WILL DISTRACT him from the victim, giving the victim a chance & time for ambulance to arrive to save her life. It only needs LARGER objects - bicycle, dustbin, whatever that he will not be able to evade easily, more so when there are others THROWING items at the SAME TIME.
Such tactics will give time for the police to arrive, usually within minutes, minutes that WILL save a life, & lives of others. Do not live in fear, but have courage, to act decisively. Each life matters, means something special to another, & we Sporeans are connected to each other by our common goal to survive & only by being together will we achieve such.