Gavin Brown has pleaded guilty to the murder of Craig Robins who was set upon in a car in Hednesford in October 2006.
Mr Robins, then aged 27, was already wheelchair-bound after being left paralysed from the chest down in a car crash nine years earlier when he was stabbed and assaulted in Lomax Road.
The attack left him needing round-the-clock care due to irreparable brain damage and he died in 2019.
Brown, who was jailed for 12 years for the attempted murder of Mr Robins in 2007, was one of four men charged over Mr Robins' death in February this year and has now admitted murder.
The 36-year-old, who is from Hednesford but is serving time at HMP Lancaster Farm, pleaded guilty at Stafford Crown Court on Wednesday and will be sentenced at a later date.
The three other men alleged to have been involved have pleaded not guilty to manslaughter and are due to stand trial on September 13.
They are James Ainsley Milligan, 41, of Rowley Close, in Hednesford; Bernie Peter Smith, 30, also of Rowley Close, in Hednesford; and Kyle Neil Smith, 33, of Pebble Drive, Blackfords, Cannock.