My advise is: Call 911 to show good faith. Worry about accomplices more than first aid. About one half of crimes involve more than one person. Do not get whacked while your attention is devoted to helping someone who moments before was trying to kill you.
Remember to keep the 911 short and to the point. "I need cops and an ambulance" and give the address, then hang up. Remember, they have a probably have a script to get information and they're recording. It's the same thing as talking to a cop, IANAL but I'm guessing anything you say on a 911 call is admissible in court.