I’ve always thought the phrase “burned out” meant, just sick of doing it, heart is not in it anymore. It sounds like your definition probably means more than that. Primary Caregivers are under a tremendous amount of stress and pressure...
Your description of the term is accurate, but incomplete. A lot of angst goes into a nurse's decision to change practice venue. Nursing specialty areas with the highest burn out rates are those where there is infrequent opportunity to see a patient recover from illness. In the ICU, when a patient survives the critical care episode, they are promptly shipped out of the unit. Oncology units and nursing homes are permeated with poor prognoses.
As noted before, I think compassion fatigue is a reasonable description for health care provider burn out. Using compassion fatigue synonymously with secondary PTSD is crap. Using burn out, compassion fatigue or secondary PTSD as a reasonable defense for murder is incomprehensible.