Asher Maslin, who brutally killed his ex-girlfriend Hollie Gazzard when she dumped him, "couldn't take rejection," police said.
Detective Inspector Angela Middlewood, of Gloucestershire Police, said: "He was a very volatile individual. He was very controlling and possessive.
"Hollie had broken off the relationship with Maslin and he couldn't take the rejection."
She added: "I don't think we believe that Hollie thought that something as brutal and awful as that would happen but she was aware that he would turn up and pester her and couldn't accept that the relationship was over."
More top news
Simon Cowell says he "felt like a bit of an idiot" after learning the Britain's Got Talent winner used a stunt double dog during the final.
An advert for luxury fashion brand Yves Saint Laurent has been banned for featuring a model with a visible rib cage and stick-thin legs.
Every school rated as inadequate by Ofsted will be turned into academies under new laws, the Education Secretary has announced.