There are any number of cases where medical staff have deliberately killed patients. If you google, you'll be spoilt for choice.
I know it's not something we want to believe happens, but if you think about it, if you're a psychopathic killer, it's logical to get a job in a hospital. You get access to vulnerable people, and questions won't be immediately asked if someone dies.
After a while someone may notice a pattern, and investigate, as appears to be the case here. But I wonder how many times it happens and nobody notices.
Also, discussion of ongoing court cases is generally frowned upon for legal reasons.