Slupinski was a psychopath who was confined within The Slab for an uncertain amount of time before E-Day. Although the exact nature of his crimes and mental stability were shrouded in mystery, it must have been serious enough for him to be confined in the Psych ward, along with the rest of the prisoners who ate mailmen and killed for a hobby - as Millton Reeve put it. Slupinski was later killed in the genocide of the Psych ward inmates. His body, along with the other casualties on both sides, were burned in the courtyard.
References[]
In the fifth & final book of the Gears of War novel series, The Slab, it is mentioned that Slupinski's cell is on the same passage as Beresford's cell, a rapist who was also incarcerated in the Jacinto Maximum Security Prison.