Joint enterprise convictions are a form of Folie à deux. The case last week is a prime example of this concept. Other famous cases such as Fred and Rose West, Ian Brady and Myra Hindley are further evidence of this psychotic disorder.
The definition from Merriam Webster is "the presence of the same or similar delusional ideas in two persons closely associated with one another." The madness of two people is shared through this condition. It usually involves one dominant person convincing others of their belief to kill.
Whether the parties in these cases would have killed on their own is a matter of debate. Many will say that they do not know how these events happened; the person must have had violent tendencies before, but several experiments show people can change their behaviour if the situation suits them.
Let us take a deeper look at the psyche of the ordinary person and answer the question, could you become a killer in the right situation?