A man with paranoid schizophrenia who stabbed two Big Issue sellers to death in a random attack on a city centre street has been sentenced to life imprisonment.

John Ward, 23, himself a homeless man, attacked Wayne Busst, 32, and Ian Watson-Gladwish, 31, close to their pitches in Birmingham as voices in his head ordered him to "kill everyone". The founder of the Big Issue, John Bird, said the case was a tragic reminder of how cuts to mental health services and the care in the community policy had led to dangerously ill people roaming the streets.

Bird admitted that the loss of the men had led him to wonder if he had been right to set up the magazine, but said he was now adding impetus to schemes to find a different way of working for sellers, including training some to provide content for the magazine's digital section and finding safer spots within shopping centres and supermarkets for others. He said: "Life on the street is getting more dangerous. The streets are filling up with people who would have been treated within a mental health environment in the past."

Ward's conviction comes less than a fortnight after the case of schoolgirl Christina Edkins, who was stabbed to death on a bus in Birmingham by Phillip Simelane, who also had paranoid schizophrenia.

Ward pleaded guilty to manslaughter of the two vendors on the grounds of diminished responsibility. Mrs Justice Thirlwall told him during a hearing at Birmingham crown court it would be 12 years before he could be considered for release, although she said he was an "exceptionally dangerous" man who might spend the rest of his life behind bars.

Peter Grieves-Smith, prosecuting, said Ward got off a train from London just a few hours before the attack on 11 January. He had left the capital after a row with his mother ended in him punching her in the face. In Birmingham he bought a 20cm (8in) kitchen knife, whisky and heroin, which he smoked.

He met Busst and Watson-Gladwish and seemed to get on well with them but commuters watched in horror as he suddenly launched his attack. He struck Busst first, stabbing him in the chest with such ferocity that the knife hit bone. Watson-Gladwish tried to help but was stabbed twice. Ward then threw away his knife and walked to a cinema, where he wiped blood from his face and hands with napkins, until the police arrived to arrest him.

The court heard Ward, originally from Ireland, had a history of violence. He was convicted of robbery and assault in 2011, receiving suspended jail terms for both offences.

While on remand in the high-security Ashworth hospital in Merseyside, Ward attacked a nurse and now has to be accompanied by four warders whenever he moves around.

The court heard that Ward had told doctors that before the attack he had heard voices that "wanted me to kill everyone". He said the voices told him to "kill random people".

In mitigation, Michael Duck QC said he grew up in Ireland in an abusive household, surrounded by alcoholism and drug-taking. Duck said Ward had started drinking at age nine, was taking heroin by his early teens and was taken into care where he was sexually abused.

"From a very young age he reported hearing voices, which was a clear marker of the very significant mental health issues he's suffered and the tragedy is it's been picked up rather later in life," said Duck.