Not only strictly emotional, not strictly mental, not strictly at the level of personality or visceral bodily reactions, not only the trauma of the unprocessed replaying itself on a loop and growing stringer until it consumes all.
At a purely biological level PTSD physically changes the brain structure. It shrinks the hippocampus (the inhibitory structure that allows you to govern your emotional responses and helps you keep the coherence of personality) and enlarges the amygdala (which makes you more emotionally sensitive and unstable). Antidepressants can help you grow back the hippocampus, but once touched by the PTSD, the amygdala never shrinks to its previous size again and heightened sensitivity and relapse are always a possibility.
But outside the biological sphere, PTSD is a whole personhood, a whole body tragedy as a consequence of encountering something so staggeringly different, an experience that just destroys all of the axioms of your previous knowledge. It is so unexpected and you’re so unprepared that you just have no emotional or mental faculties to recover or rationalize. It hit such a blind spot that nothing you know to be true can’t explain or compensate for it. It usually happens when encountering malevolence in any form, because malevolence is not our natural state and yet it exists as the dipole of life. This might be malevolence coming from the outside world or malevolence you found in yourself once you truly see what you’re capable of and couldn’t once even imagine doing. It is not a prerequisite, but there’s a tendency that those who experience were (un)willingly blind to some segment of life which leaves them naive and opened to falling apart because they’ve had things coming in filtered until now, cherry-picking what they liked of the facts, never really integrating the shadow part in the whole.
There are things outside of the known domain that you need to know to be able to handle the world without being crushed by each new discovery. The optimal way of being is between the ordered world you feel comfortable in and explorations of the unknown. One or the other alone won’t suffice. This is why you shouldn’t put all of your eggs neither in the too liberal nor to0 conservative side – politically or life wise. Nothing is completely black nor completely white and all happens within a context.