You could argue the Holocaust was beyond brutal, and 1/6 is certainly not of the same magnitude, but the Holocaust's existence doesn't mean describing a beating as "brutal" is inappropriate. It's pretty standard usage.
People get brutally attacked on a daily basis. Genocide is a bit less common.
I think it actually would be inappropriate (or at least more than a little inaccurate) to describe a beating as brutal when genocide has already been brought into the conversation. Talking about a beating on its own, brutal could absolutely be appropriate.
It's as if we have a scale from 0 to 10, where each number maps to an adjective. To me, brutal is towards the end of that scale. Everyone seems to have forgotten how to use the words on the 3 to 7 range.
OK, what other words are we not permitted to use to describe something once the Holocaust has been mentioned? Am I allowed to describe something as "bad"?
On the contrary, bad seems pretty appropriate, because there are words that mean "worse than bad". "The worst" would be an example of something that would not be appropriate, unless it is actually worse than the holocaust. To me, brutal is basically a superlative.
I have the same reaction when I see people describe something as "extremely x". I don't often see things described as "extremely" whatever and think, "yep this person has a decent grasp on reality". This may just be something I'm sensitive to for whatever reason.
People get brutally attacked on a daily basis. Genocide is a bit less common.