Sorry, but it wasn't "mainly Jews".
According to Rummel,
(http://www.hawaii.edu/powerkills/NAZIS.CHAP1.HTM)
the Nazis exterminated on the order of 22,500,000 people. The Jews didn't even constitute a majority of those so killed. The ethnic group that suffered the worst under the Nazis was NOT the Jews--but the Slavs, with the Slavs suffering almost twice as many dead--~10.5 million as opposed to ~5.3 million.
http://www.hawaii.edu/powerkills/NAZIS.TAB1.1.GIF)
Most of the Nazis Slavic victims were either soldiers or resistance fighters killed in battle or civilians killed in reprisals and raids, like the famous one at Lidice.
Only a small fraction of Slavic victims of the Nazis were gassed in the camps.
For every person of Jewish descent killed in the camps, 1.3-1.4 persons of non Jewish descent were killed.