Brianna Ghey: How could someone so young turn into a killer?

As the two teenagers who murdered Brianna Ghey are sentenced to life in prison for at least 20 years, Rob Smith takes a look at how two seemingly ‘innocent’ teenagers became killers, concocting a kill list, luring Brianna into a park and stabbing her, in a "frenzied and ferocious" attack, 28 times.