Having been trained as a combat lifesaver(Its a fancy word for moderate first aid on the battlefield. Basically I could help to stabilize someone until the medic got there, so, not too fancy.)-I would like to think that I would act first and think about negative repercussions later. Let them sue me for a few bruises associated with saving their life-at least I'd be able to sleep at night instead of being gutted by the guilt of a dead person that I was able yet unwilling to help.
However-part of my training involves talking to the person to see if they are conscious first. This is crucial because the person might be in shock and that can contribute to violence to try and defend themselves. So prior to beginning any basic first aid, I would establish the mental condition and I would put the emergency dispatcher on speaker to alert them to the emergency.
If the person were conscious-I would introduce myself, ask if I could help, tell them they were going to be fine-even if it looked as though they might not make it. This prevents the person from freaking out and damaging themselves further and helps me maintain control of the situation. I would put pressure on the wound if the knife was not present. If it was, I would not attempt to pull it out-but if it was bleeding-I would put pressure around the knife to stop the bleeding.
If the person was unconscious, I'd still talk to them-I would let them know exactly what was happening and what I was doing, that help was on the way, etc. I would check for breathing, bleeding, any broken bones sustained from falling over wrong, etc. and prep them for evac. Above all-I would listen to the dispatcher unless the situation dictated otherwise.
If the person had an open chest wound I would try to dress it and place them on their side with the wounded side pointed down. (placing them on their unwounded side could hinder breathing.)
If the person needed a tourniquet (say they got stabbed in the bicep or thigh,) and I was able to make one-I would write down the time I applied the tourniquet on his forehead in pen or marker, as well as any time when I needed to tighten it. (tightening is less likely as the individual would be picked up fairly quick.)
If anyone else was there to help, I would have one willing person keeping an eye out for anyone shady-to prevent any more attacks, as well as looking for the ambulance so that we could get help for the individual as soon as possible without a lot of hiccups.
I am no health professional...but this is what I would do, having been trained to do so.