I just wonder that if I coward of a street fight, how good will I be on an actual revolution or in an apocalyptic scenario? Also, I wonder if not fighting bullies gives them ammunition to get away with their bad behavior and or reinforces it?
I know that in our current day and age, one might refuse to fight and just let the police deal with that. But if the collapse happened and we were in Mad Max, is not fighting a good strategy?
What is the philosophy on fighting? And on walking away?
As a rule of thumb, if you're unsure whether you should fight, you should most likely walk (or run) away. Even top of their league fighters are still at a significant risk of death or permanent injury and you shouldn't flounder into some fight if you don't know what you're doing or why you're doing it. Since I assume from the title that you mean physical fight, you should also always consider the possibility of the assailant being secretly armed, or that others might join in.
As others pointed out, most people wouldn't be fighting (and probably physically can't) unless shit has really hit the fan (in which case you're not gonna have much choice), and the proper way to do it is through organised people's militias. I always recommend learning some martial art because it's actually very cool for your health no matter if you get into a fight, but also because it helps you understand the risks of confrontations, how to keep your cool, and maybe even how to de-escalate some situations safely.
Also guns are long-range, hyper-accurate, supersonic pellet-launchers that can pierce flesh and bone, no matter the size of the meat lump we call humans. Doesn't really matter that much who is shorter or taller if one of them is packing one. Blunt force weapons (hammers) are also really effective at causing a lot of pain, if not outright killing, even if the person using them is not particularly strong.
TL;DR: Don't lone wolf it.