It requires a legitimate fear for one's life. The threat has to evidence intent, capability, and opportunity. This person was in a bad spot, surrounded by obviously hostile people. She was legitimately in a fearful situation. And oh heck yeah, if one of them pulled a knife to threaten her she would have been clearly justified in stopping that threat with deadly force.

It's a bad, bad deal, and it's completely outrageous our politicians have let this gain so much steam.