Alleged double killer Russell John Tully had claimed to have been suffering from painful pus-filled boils that inflamed his armpits and groin.
Dermatology experts believe Tully, 48, was most likely suffering from hidradenitis suppurativa when he allegedly walked into Ashburton's Work and Income centre and gunned down three people.
Office workers Peggy Noble, 67, and Leigh Cleveland, 55, died in Monday's shooting.
Lindy Curtis, 43, was critically injured and is recovering in hospital.
Tully had vented frustrations to the Ashburton Guardian about being unable to find a place to live after returning to his hometown.
In the August 2 article, he said he had "come home to die" from what was described as "an unidentified skin disease which caused boil-type lumps if untreated and tightened the joints causing limited movement".
The malady had placed him on a disability benefit and was likely to worsen. His doctor said he should have a mobility scooter.
His symptoms were consistent with hidradenitis suppurativa, according to dermatologists.
The chronic condition causes inflammation of apocrine sweat gland-bearing areas - underarms, breasts, inner thighs, buttocks, and in the groin and genital region.
It leads to painful and recurrent boils and abscesses.
"It can be quite difficult to heal and very painful," said one dermatology expert who declined to be named.
Professor Amanda Oakley of Waikato Hospital's department of dermatology said the condition could be very debilitating.
In extremely rare cases where sufferers have an abnormal immune system, it can even be fatal.
Treatments include antibiotics, the surgical drainage of abscesses, or even surgical removal of affected skin areas. Healed areas are marked by thick scar tissue.
Sufferers are advised not to smoke, wash carefully but often, to eat well, wear loose-fitting soft clothing, and avoid using deodorants or perfumes on affected areas.
A symptom of severe cases is depression.