ELI5 Why are some Halloween costumes deemed "too offensive"?

Explain to me why it would be entirely inappropriate to wear an SS costume to a bar costume party, but acceptable to dress as Ed Gein, or a pervert in a trenchcoat, or a used tampon, etc? Full disclosure: I have an imitation SS Obersturmführer uniform that was created by a costume maker out of San Francisco from an East German military uniform, very similar to the one Ian McKellen wore in the 1998 film Apt Pupil with boots, cap, etc. No red armband, but it does have the eagle w/ swaz sewn on the sleeve.

I wore it to a bar Halloween party around 15 years ago and the response was relatively positive; high fives from guys, people coming up to me speaking German, and surprisingly women were really into it. I am being told now that if I wore I would probably get my ass kicked. I don't get it, it's just a costume.