Strangely more common than gets reported much less caught. A lot of research has been done on this in terms of profiling healthcare serial killers - they are predominantly nurses and female. Why? It's the burning question
My take, it's the most overworked, stressed out and underappreciated worker in the hospital setting. Nursing profession also attracts particular personality profiles that have been researched extensively - obviously patient advocacy & empathy are strong traits but so are other underlying ones that can manifest themselves in unimaginable ways
It's a tough job, tougher than just about any I have come across and you have to be tough to thrive in it but it also attracts a lot people with underlying anger that is often well hidden