"That 70s Show" star Lisa Robin Kelly was killed by lousy treatment and an incompetent staff at the rehab center where she was found dead ... according to a wrongful death lawsuit filed by the late actress' husband.
Robert Gilliam is suing Pax House in Altadena, CA claiming Kelly's "treatment and care were improperly administered" ... and that she was neglected for "an unreasonable amount of time" by workers who weren't fit for the job.
TMZ broke the story ... Kelly died Aug. 15, three days after she was admitted to Pax House. The coroner ruled it an accidental death from multiple drug intoxication -- but never identified which specific drugs were in her system.
Gilliam claims in court docs -- obtained by TMZ -- Kelly was last seen alive by Pax workers sometime during the night of Aug. 14 ... and it wasn't until several hours later that she was discovered dead in her room.
He says his wife's death has caused him loss of love, affection, comfort ... and financial support.