Philosophically, established moral codes are the opposite of arbitrary, they have been codified over generations based on the previous peoples lived experiences.
"Don't assault someone just because they called you a bitch" isn't some new white colonial imposition, it's a small part of how we've been able to maintain larger groups of humans all living and working together for common goals.
You can call laws arbitrary, as they often are, but shared moral values are some of the least arbitrary concepts humans can grasp.
Further, the American system of white supremacy isn't upheld by shared societal morals it is in direct opposition to those morals. That's why the framers of the constitution were so flummoxed by the issue of slavery, these self-evident truths were not compatible with the fact that slavery was accepted normal and ethical.
Nepenthe is just saying things that she thinks sound smart and revolutionary, but she sounds like an idiot. She's just playing Malcom X Madlibs.
To be fair the larger concept of proportionality of justice, or proportional response to an offense is old as society, but the specifics of it vary wildly.
Honor killings for example were a thing not too long ago (and still are in some places).
So you could argue a punch to the face is rightful retribution for a perceived offense to your honor.
That is, if you're ok living in a barbaric society with medieval values. 
To continue down this fun tangent:
Well, even within the moral frameworks that allow for honor killings they are often seen less retributive and more restorative; they are less ad hoc and more ceremonial, having to follow the previously established processes within a religion or other larger controlling superset of moral codes. Of course, that doesn't ever stop people from carrying out this kind of justice in a very ad hoc way with no repercussions because people will warp whatever framework they're following to suit their needs. Punching someone for calling you a bitch may not have always been illegal, but in many times across many cultures it would still be viewed as immoral or uncouth.
For a white colonial example and a non-white colonial example; killing someone in the street for besmirching you would still be seen as a criminal act within 16th Century England, the structural "duel" was not made illegal until the 17th century and even then was still practiced without repercussion for decades after. Even now men in white colonial society will arrange for a time and place to fight and restore honor, even if we don't call these duels the origination is the same.
Today in Pakistan, honor killings are still carried out regularly and normally these killings are done by close family members not because the women have made someone mad, but because some previously established illicit activity has besmirched the honor of the entire family and the only way to restore it is through honor killing which must be carried out in specific ways.
What I'm saying is that Jason Derulo should have found out who this dude was and made this guys Uncle splash him in the face with Acid.
Nep refusal to accept "your" arbitrary moral codes is less about colonial supression and more about her own lack of empathy toward other humans.
