So this crazy sicko was ranting and raving, spewing trash - the story doesn't indicate he pulled the knife until someone intervened. So - when the "good samaritans" intervened, did they verbally challenge him, prompting an immediate murderous response, or did they lay hands on him before he pulled the knife? In which case, I can see a shyster making a self defense claim. (Or trying to.)
If the former, the guy should fry; if the latter . . . it shows the dangers of physical confrontation with a lunatic when you're not a trained & equipped LEO with backup.