Valdo Calocane, who killed three people in Nottingham, is eligible for hundreds of pounds in benefits.
The 32 year old is entitled to claim thousands of pounds in state benefits a year through Universal Credit despite being detained.
He was sent to the high-security Ashworth Hospital in Merseyside for killing 19-year-old students Barnaby Webber and Grace O’Malley-Kumar and school caretaker Ian Coates, 65.
The original charge of murder was downgraded to by the Crown Prosecution Service (CPS) to manslaughter as a result of his diminished responsibility.
Barnaby Webber, Grace O'Malley-Kumar and Ian Coates were stabbed in the street and killed in the early hours of June 13Composite: PA; Huntingdon AcademyNow, the mother of one of the victims has called on the government to review the law into benefits payments.
Barnaby's mother Emma told The Telegraph: "This is truly devastating to hear and it adds yet more layers of grief and a sense of injustice to those of us left behind.
"Whilst we are desperately trying to process our enormous grief, battle to try and find a way forwards to return to work and support our families, this vicious monster not only has tens of thousands of pounds of taxpayers’ money spent to keep him inside, he can also amass a small fortune of state benefits.
"How can this possibly be fair? This may be the law, but we strongly suggest the law is wrong, and urgently needs to be reviewed."
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Emma Webber made a statement outside Nottingham Crown Court after the sentencing of Valdo Calocane
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Calocane is entitled to claim state benefits because the judge ordered that he should be detained in a high-security hospital under sections 37 and 41 of the Mental Health Act.He can only be released if the Justice Secretary deems him to be no longer a risk to the public.
The benefits are designed to ensure that his dignity is maintained while in care, with funds for items such as clothes, food, books and electronic equipment
Former justice and home office minister Sir Mike Penning said: "How can it be right that someone who is a killer and guilty of such a horrendous crime can still get benefits at the taxpayers’ expense?
"Justice is not being done, and not being seen to be done. We desperately need to put the victims first. This needs to be reviewed immediately as part of the new victims’ law."
Calocane is being held at Ashworth Hospital by Mersey Care near Maghull, Merseyside
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A Mersey Care NHS Foundation Trust spokesman said: "All our patients within Ashworth High Secure Hospital are entitled to state benefits depending on their personal circumstances, which are paid by the Department for Work and Pension (DWP) according to their rules and regulations."
A Government spokesman said: “"We have been clear that questions need answering in all aspects of this case.
"Entitlement to benefit is, by law, dependent on sentences handed out, and the Work and Pensions Secretary is looking at the specifics of this case."