If you did that then after one or two generations, regression to the mean would set the average IQ right back to where it was (almost). If you eliminated enough of the left tail over several generations to actually change the average to a stable higher value, then the right tail would be extended.
Like I said I'm not commenting on the effect of the Holocaust because I don't know anything about it.
If UberHitler kills everyone with IQ<100, that raises the average IQ without increasing the number of people with high IQ. After a few generations, you are back to a Gaussian with a smaller variance (you lost some genetic diversity) and a slightly larger mean, which means that at some IQ level that is sufficiently high you have fewer people with that IQ .
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.