One of the facts about modern (World War One and beyond) infantry
combat is: if you leave people in combat for long enough they
go mad. Eventually 98% of participants will become certifiably
insane. The other 2% were certifiably insane already.
There are certain phases that participants go through.
through the "Period of Maxiumum Efficency",
and into "Combat Exhaustion" (broken into several sub-phases):
"Period of Overconfidence / Hyper Reactive",
then "Emotional Exhaustion"
then "Emotional Exhaustion / Vegetative".
From "Darkness, Be My Friend" through to the end of
the series, the team is basically bouncing around between the
various states of "Combat Exhaustion"