But if you've been slaughtered and raped, you can call your oppressor whatever you want.
Most of the people from the countries that Japan colonized openly want/beg for westerns colonialists to "come back".
American WASPs and Burmese people (among many others) both have a history of their grand/great grandfathers being brutally tortured circa WW2 by the average Japanese persons great grandfather.
As it turns out, a slur that's literally just removing a couple letters from your countries name isn't all that bad in the grand scheme of things. I'll stop using it when they stop calling me "Jingai" and "Gaijin" because I had the audacity to put my seat down on the Shinkansen without asking the entire train if I was going to lose face by doing it.