In 1951 he met Joe Orton, a fellow RADA student, and they formed a relationship. In 1959 they both began stealing and defacing library books, for which they were sent to prison in 1962.
The relationship came under strain with Orton's success as a playwright. on 9 August 1967 Halliwell killed Orton with hammer blows to the head and then took an overdose of sleeping pills.
In 2014 there was an exhibition at Islington Museum, featuring Halliwell's collages based on the defaced library books. In 2021 Halliwell's Cats montage screen was displayed in Islington Museum.