There’s a wiki article on the subject of nurses who kill their patients. It contains some general speculation on motivations.

The motivation for this type of criminal behaviour is variable, but generally falls into one or more types or patterns:[4]

Mercy killer: Believe the victims are suffering or beyond help, though this belief may be delusional.
Sadistic: Use their position as a way of exerting power and control over helpless victims.
Malignant hero: A pattern wherein the subject endangers the victim’s life in some way and then proceeds to “save” them. Some feign attempting resuscitation, all the while knowing their victim is already dead and beyond help, but hope to be seen as selflessly making an effort.