It's hard to put your finger on it... Which I believe is why it's drawn such worldwide attention.
I think the key point is as I said, when does the point come when all the circumstantial evidence reach a critical point of being unable to be ignored? On their own, everything can be explained away, but when it's looked at as a whole, it becomes significant.
We're talking about the medical records she took home, the asterisks she put in her diary the day the babies died/collapsed, the scribbled post-it "confessions", searching up the families on social media much later, her being the only nurse who was present for every single incident, for example.
Any one of those on their own doesn't hold enough weight, but added together, they paint an ominous picture.
And let's be honest, if she hadn't been young, fairly pretty, white and British-born, the whole case wouldn't have attracted half the attention it has. So many people looked into the case, solely because she "didn't look like a killer."