On Killing further develops the idea [0] by looking at a wider set of battles across time and, crucially, finds that by adapting training methods, the kill rate went up to beyond 90%. This then appears to come with higher rates PTSD.

0 - https://en.wikipedia.org/wiki/On_Killing

On Killing also has serious issues with credibility

it relies on SLA Marshall's dubious work, and several other examples it uses are difficult to take seriously.

it's similar to Freud, where there are shreds of truth but not really universally true or applicable.