In an exclusive interview set to air on Channel Seven's Sunday Night, Karlie Tyrrell said she 'felt like the worst mum in the world' after her two-year-old was snatched from his foster grandmother's home in September 2014.
'Whoever has him needs a bullet,' an emotional Ms Tyrrell said.
It is believed William was abducted from the home in Kendall, NSW, while playing with his sister in the backyard.
In the brief clip ahead of the program, Ms Tyrrell is also quoted saying 'they were responsible and they failed'.
It is not clear who she was referring to, but it is believed she has strong feelings about William's foster parents and the Department of Family and Community Services.
It was revealed last year to the public that William was in foster care at the time of his abduction, but police have said this is not linked to the disappearance.
Earlier this year, the 29-year-old mother appeared before Burwood Court after being arrested for a range of offences, including spitting on a police officer.
She entered a guilty plea for assaulting an officer and using offensive language in public after being arrested at Ryde shopping centre in December last year for 'acting aggressively towards customers'.
She will be sentenced in March.