Can you legally open fire to help someone in distress?

A man was caught on camera stabbing a woman to death on the Loop 101 near Peoria Ave. A witness opened fire, hoping to subdue the suspect. Can you legally shoot someone who isn't posing a threat to yourself as long as they seem to be harming someone else?
Published: 10:29 PM MST March 25, 2019
Updated: 10:27 PM MST March 25, 2019