Actually, I just watched a documentary a few weeks ago (I'll try to round you up a link when I can remember the name of it) on the weird beliefs of the Nazi elite and particularly Himmler and the SS. Apparently Hitler and Himmler believed that Germanic people came from a superior (white) race, that defiled themselves in the ancient past by interbreeding with "inferior" races. They evidently believed that by wiping out inferior races, the "purity" of the superior Germanic race would improve, and they'd gain back their superpowers. Apparently they had special powers before interbreeding with inferior races. There was actually a very methodical attempt at "ethnic cleansing" by the Nazi elite.
According to their weird beliefs, Jews were considered the most inferior of all the races and wiping them out completely was a primary goal of the SS. Killing all the Jews was a necessary "final solution" to their goal of ethnic cleansing.
It's not like the Russians really fared any better at the hands of Germans mind you, and they were also slaughtered by the millions, but there was actually a methodical attempt by the SS to completely wipe out the Jewish population from the face of the Earth.
As you note, other handicapped individuals, mentally ill patients, etc, all suffered the same fate of outright extermination, but the concept of ethnic purity and the intent to eradicate the Jewish population were in fact primary goals of the SS. They were very much intent trying to "purify" the DNA in their twisted minds.
When I can remember the name of it, I'll find you a link. It was something about secret occult beliefs of the SS or something like that. Himmler was apparently a primary orchestrator of the 'final solution' aspect.