My current area of research is in regards to war crimes in the Far East, focusing on the legal issues and doctrines surrounding prosecution of "low-level" war criminals. I'm sure someone with far greater knowledge would be able to give a response that directly answers your question, rather than the broad brush stroke below.

The sources that I have available do not deal exclusively with guards at death camps in the long term, but rather with members of einsatzgruppen, who were used to liquidate "undesirables" during the Eastern campaign, and some sources re mass executions at camps.

While the diagnosis of PTSD in military medicine is relatively new, and didn't exist as a recognised diagnosis at the time, it certainly was an issue for those who were art of the Einsatzgruppen, who were either SS or Wehrmacht soldiers tasked with field executions. Those who were tasked with executing suspected partisans, local leaders, Romany, and Jews were often allocated large quantities of alcohol both before and after the executions. Using large numbers of soldiers to take part was a means in which to spread the guilt of the atrocity across a large group, in order to minimise personal involvement.

Himmler, head of the SS, found that not only were mass shootings inefficient, and detracted from the war effort (in regards to logistics and some manpower issues), they also took a heavy toll on the men who took part. Though I can't find the source right now, I believe that Himmler thought it inhumane to place such a burden on the German soldiers.In fact, Himmler made sure that SS men who took part in the massacres and suffered ill health, mental or physical, was to receive additional care. Also, later in the Eastern European campaign, efforts were made to use local auxiliaries to execute 'undesirables', as many soldiers suffered long term issues, especially when killing women and children. The transition to gas chambers was a means of limiting both the logistical issues with executing by firing squad, and also a means of limiting mental health issues that accompanied taking part in the massacres.

One of the most interesting things that I find about researching war crimes is the way in which the perpetrators are dehumanised, similar too, though obviously in a far lesser form, than what they themselves did to their victims. Those who committed these acts were human, and did have mental health issues, and even feelings of guilt, for the crimes they committed. This is certainly NOT to say that it was a reprehensible ac, or deny the existence of what occurred.

Sources;

Richard Rhodes- Masters of Death: The SS-Einstazgruppen and the invention of the Holocaust (2002)

Ben Shepherd- Hitler's Soldiers: The German Army in the Third Reich (2016)

Ben Shepherd also presented a paper which I attended at the University of Western Australia on 3rd August (just this week), which paid some slight attention to what I have responded with.