In the ordinary consciousness, one deserves what one receives in keeping with the causal laws - what we sow, we reap.<quoted text>
Sounds more like anti-semitism than Karma.
What exactly did Jews do to deserve their fate?
Nothing happens without a cause.
The causation can be complex and invisible to the surface mind.
So, what underlying cause led to the Holocaust?
If not past birth karma then was it something in the heredity or was it something to do with the karma of the victims in the present life?
There has to be a causality at the psychological, emotional, physical and/or heredity levels.
BTW, what do you understand by karma?