My response is about the generalized statement you're making, and not particularly about the bus driver's response, as there is hardly a well enough view of what she did for me to judge his actions.

Sex and gender do not have an influence on how I deal with a physical threat to my well-being. Suggesting that I "should never" hit a certain kind of person because of some kind of disadvantage, including the context in which they have physically assaulted me is both ridiculous and also not your place. People should still hold value in the fact that their aggressor is [insert an adjective]? Men should never hit a woman because [...]. A bigger person should never hit a smaller person because [...]. Now, leave those bracketed descriptors the same and replace the target nouns with "aggressor".

People defend themselves however they can.

What I do know about the video, is that this irate woman physically assaulted a bus driver in some fashion, and also put the entire bus at risk by doing so while he was driving. She was a dangerous person to have on the bus and a threat to his personal safety. Did that warrant a brutal response? Probably not in the fashion he acted out, but as I'm sure many can attest: 1) Not everyone is sound of mind enough to guide an "appropriate response" when the adrenaline in your body rushes over you when being attacked, and 2) Not everyone is going to be confident enough in their ability to alternatively neutralize a threat, such as subduing or some other measure of non-violent prevention.

My only ethical responsibility, as a [...] responding to a physical threat from a [...] who has the disadvantage of being [...] is to protect myself.