The Dirlewanger Brigade, led by well-known alcoholic, child rapist, and former concentration camp inmate Oskar Dirlewanger. The brigade consisted of convicted felons (murderers, rapists, arsonists, violent thieves) and the mentally ill from Germany's prisons and concentration camps, patched together by rejects from other units. Dirlewanger himself was described by one historian, Richard Lucas, as "a sadist whose brutality was well known ... one of those degenerates who, in saner days, would have been court-martialed out of the German army." What happened in Warsaw in 1944 at their hands alone, even in the background of other German atrocities that included razing the city to the ground and after two years of the Dirlewanger Brigade terrorizing what is now Belarus(and you should read about some of what they did there, too), should seal their place in infamy. Alone.

http://www.historyanswers.co.uk/history-of-war/oskar-dirlewanger-the-ss-butcher-of-warsaw/

They were so bad that even many in the SS thought they were a disgrace and didn't want anything to do with them. Friedrich Wilhelm Kruger, the SS (!) chief of police in Poland(!!), stated in early 1942 that if they weren't kicked out of the General Government, he'd arrest them himself, which is how they ended up doing anti-partisan operations in the USSR. SS judge Georg Morgen actually tried to bring Dirlewanger to book for his wanton brutality and corruption. But of course, Dirlewanger had some friends pretty high up in the SS hierarchy: Morgen ended up getting demoted and sent to the Eastern Front as punishment for his gall. Numerous Wehrmacht and SS commanders likewise attempted to disband the brigade, but all this was futile due to Dirlewanger's patrons within the SS and the Nazi Party bureaucracy, who incessantly intervened on his behalf.

I know also that the Germans were taken aback at the brutality of the Ustase in the former Yugoslavia against Serbs. I also think they might have been at how the Romanians dealt with Jews as well, in spite of their own goals in Romania, but I don't remember.