I don't really care for Cassie or Violet. Cassie is a narrator that tells me too much every step of the way and Violet was too condescending. If she was an actual person, I think she would have been the reason people voted Leave.

For someone that only looks at the facts, Violet did an awful lot of assuming herself.

Fact is four people died after eating soup they bought at a street vendor. Assumption is "If you're trying to poison as many people as possible, then you would want to go as long as possible without being noticed." Assumption is a serial killer would not use this method. Assumption is the murderer must have had one specific target in mind. Though in the narrative Cassie makes these assumptions, Violet is encouraging these assumption, suggesting she has made the same assumptions and not showing any facts to back up her assumptions.

And never did anyone ask: how did the killer get the poison in the soup? Who had access to that kettle?

Violet and Cassie go on to find evidence to back up their assumptions. Including breaking in to the office of one of the victims. Any evidence found there would not be usable in court as it is obtained illegally. Apparently, that doesn't bother Violet.

In the end, the whole thing comes down to the murderer admitting the crime. There is very little evidence (I mean facts here) to back up the murderer did it. Yet everyone is happy. I'm left wondering what would happen if this case is taken to court and the murderer pleads innocent.