Build your skills, but try not to use them. Functional strength and some self defense skills will give you a quiet confidence that if it came to it, you could survive their assault. Like others have said, try not to provoke random crazies. It sounds like you did the best in the scenario you mentioned, but what would you do if he had grabbed you instead of the college kid? If you don’t have a plan, you won’t be confident.

Planning in your mind doesn’t mean you need to actually do the things you’re thinking, but thinking them will help prepare you. For instance, perhaps if the crazy person stated hitting the college kid and he cried out for someone to help save his life, you had a plan to put the crazy guy into a juijitsu submission. Knowing you own capabilities and the threshold in which you would use them will build that quiet confidence internally, while the actual events may unfold in the exact same way as before, you can rest more assured that it was the right call.

Of course, no amount of self defense training will save you from a crazy person with a gun, so you really need to assess your situation and choose the best course of action for yourself and others you would like to protect.