Death nor violence does not have to be inherently traumatic. I would be very surprised if 100% of people back then had degrees of PTSD (and that's not what my comment was meant to imply).
A working definition of trauma: "trauma can be defined as any unresolved autonomic nervous system response. It’s about the nervous system’s response to an event, not necessarily the event itself. Events can affect each of us very differently."
The context in which these events happen plays a huge role in whether or not someone develops symptoms of PTSD afterwards.
Spiritual beliefs can play an important role in acceptance of death and therefore it not being a terrifying experience.
My therapist had me read a book called The Highly Sensitive Person by Dr. Elaine Aron after I told him I was struggling with things I felt a “normal” person should be able to handle just fine. The book made a lot of interesting points about what it means to be sensitive (broad sense of the word) to the world around us.
I've heard it hypothesized that ADHD is adaptive/selected for in some contexts, because people with it are more resilient to stress, i.e. it takes more to make them scared/shocked/traumatized, just like it takes more to excite/motivate them. And so such people can respond "normally" to stressful situations where the other people around them tend to freeze up, thus potentially saving the rest of the group.
I thought when someone experiences trauma, it makes their brain sensitize to preceding stimuli and unconsciously produce a strong response, in an attempt to avoid similar trauma. This would increase survival odds in life-threatening environments. The problem being that heightened sensitivity to similar stimuli and automatic strong responses are usually detrimental to living a safe, modern life.
It's possible that if you grew up in an environment in which death was commonplace, but you experienced, or even just believed in, a sense of agency, you'd experience fewer manifestations of grief or PTSD than we'd consider typical now.
Though perhaps not. I suspect many older psychological disturbances -- nervous exhaustion, mental breakdowns, melancholia and nostalgia (in their original senses) would map strongly to present notions of PTSD and depresion.