That day in the forest was traumatic for Reserve Police Battalion 101. They didn’t like shooting people in the head all day. A few of them asked for and got transfers.

Christopher Browning continues:

The problem that faced Trapp and his superiors in Lublin, 
therefore, was not the ethically and politically grounded oppo- 
sition of a few but the broad demoralization shared both by those 
who shot to the end and those who had not been able to 
continue. It was above all a reaction to the sheer horror of the 
killing process itself. If Reserve Police Battalion 101 was to 
continue to provide vital manpower for the implementation of 
the Final Solution in the Lublin district, the psychological 
burden on the men had to be taken into account and alleviated.

In subsequent actions two vital changes were introduced and 
henceforth — with some notable exceptions — adhered to. First, 
most of the future operations of Reserve Police Battalion 101 
involved ghetto clearing and deportation, not outright massacre 
on the spot. The policemen were thus relieved of the immediate 
horror of the killing process, which (for deportees from the 
northern Lublin district) was carried out in the extermination 
camp at Treblinka. Second, while deportation was a horrifying 
procedure characterized by the terrible coercive violence 
needed to drive people onto the death trains as well as the 
systematic killing of those who could not be marched to the 
trains, these actions were generally undertaken jointly by units 
of Reserve Police Battalion 101 and the Trawnikis, SS-trained 
auxiliaries from Soviet territories, recruited from the POW 
camps and usually assigned the very worst parts of the ghetto 
clearing and deportation. 

Problem solved. The police had to round up the Jews for transport, but they didn’t have to do (all) the killing. They had help with the rounding up.

In fact they still had to do a lot of killing, because anyone who couldn’t march to the train station was shot immediately. But apparently it was enough less to make the difference.

In short, the psychological alleviation 
necessary to integrate Reserve Police Battalion 101 into the 
killing process was to be achieved through a twofold division of 
labor. The bulk of the killing was to be removed to the 
extermination camp, and the worst of the on-the-spot "dirty 
work" was to be assigned to the Trawnikis. This change would 
prove sufficient to allow the men of Reserve Police Battalion 101 
to become accustomed to their participation in the Final Solu- 
tion. When the time came to kill again, the policemen did not 
"go crazy." Instead they became increasingly efficient and 
calloused executioners. 

Which is pretty horrifying when you think about it. It means if the horrors are at a distance, we don’t care about them. Even if we know all about them, we don’t care about them unless the blood is actually spattering into our faces.

Lifton writes that the doctors in the extermination camps were concerned about one thing: how to dispose of the corpses. There was a huge consignment from Hungary at one point and the crematoria were completely overwhelmed, so the corpses had to be incinerated in trenches outside. How do you do that? It’s a technical problem. That’s what the doctors talked about. They criticized each other for coming up with bad solutions.