How far does this go?

Consider the wide diversity in our world. Many different faiths, creeds, axis of preference.. most of which have been persecuted in some way in the past. Many of the different European colonist groups that came to this country came to avoid persecution, and the resulting American culture has persecuted native americans, irish, italians, jews, hispanics, african-americans... and the list goes on.

If we ban any activity that offends, or could offend anyone, then the eventual extension of that is banning much of our activity. Yet we cannot allow all activities, because some are intended to offend, as made clear by the actor's speech and other actions.

So where do we draw the line? For me personally (and I have no grounds for saying this should be your metric), it is where malice is clear to me. If it from ignorance (the college student who wear's a witch's hat, offending Wiccans), or from prevailing custom (skimpy halloween costumes, offending some conservatives), then I can't ascribe malice to the costume. If someone wears a Nazi SS uniform, a reasonable person could assume the wearer intended malice. Even so, they should be asked, to confirm malice before punitive action, and informed if malice is not clear - ignorance is still a possibility.

One could say any costume that can be labelled cultural appropriation is evidence of malice on some level, and should be avoided. What does that mean though? Perhaps it would mean avoiding all costumes that do not depict non-human monsters? Would that avoid cultural appropriation, and the possibility of offense?

No. Because a culture's monsters are often unique to the culture, and what one culture might regard as a monster due to some metric, another might worship or praise.

The appearance of half-man / half-animal for example, another might include worship. "Ganesha or (Ganesh) is the elephant-headed god in Hinduism. He is the son of Shiva and Parvati. Ganesha is a very popular god in Hinduism, and is one of the most worshipped. " - Wikipedia.

Another example was mentioned before, the prevalence of "Witches" in pointy hats and warts at Halloween, which fall under the label of monster according to many, but has little resemblance to Wiccans, other than the label of Witch.

So where does that leave us? It would seem that dressing up as anything other than ourselves, or our own cultural heritage is out.

Still not enough, because each person's take on their cultural heritage is different. According to Ancestry DNA I am over 70% scandinavian, and I've traced our family to Vikings. Suppose I dress up as a historically accurate by most accounts Bear-shirt (ber-serkr, aka Berserker) for Halloween? That could still offend someone who has a different view of their own Viking heritage.

So we are now at not being able to dress up as anything other than ourselves, if we do not want to offend anyone.

Even that runs the risk of offending. In some cultures certain colors or decorations (clothing with a dragon with 7 claws was only for the Japanese emperor in Japan during certain periods; white is the color of death in some asian cultures; all black makes some people worry about your mental health due to similarities with clothing worn during some terrible active shooter tragedies).

So we cannot avoid the risk of offending.

For me, it means I tend to turn a blind eye to anything that does not exhibit malice, even that which personally offends me. I will not chastise for a typical native american costume, unless the person adds deliberately offensive elements. I won't chastise for the drunk Thor costumes. I won't chastise for the Witches with pointy hats and warts.