The Germans could have been a whole lot rougher on civilian populations they tended to consider their own (like the Dutch) for starters, the German commander at Paris could have followed Hitler's instructions to level the city if it was in danger of capture, an earlier Holocaust could have been undertaken.
The problem is the earliest stages of it were inefficient as Blair touched on, I don't really see a way to avoid it though, this was a new process, and the industrialized mass murder only came about because people realized the earliest solutions were infeasible on a large scale, and it was giving Einsatzgruppen regiments serious problems with morale.
Getting the Germans to abandon Germanization as a policy could increase the body count rather significantly, the problem with that being that they'll lose the support of anyone inclined to take their side, which will be a war loser.
I think, at the end of the day, the best way to make the Nazis worse is give them half a chance to implement things like Generalplan Ost, Calbear covered in all its horror in his timeline.