Police said Jack Murray refused to drop the knife before he was fatally shot in December.
Murray's family said he was having a mental episode and needed help, not deadly force.
"Why? Why? He never hurt anybody," Murray's mother Donna said.
Amid her grief, she's demanding justice.
The lawsuit says Elk Grove Village police violated their own deescalation policy when they shot and killed the 24-year old, while responding to a distress call.
The wrongful death complaint was filed Wednesday in Cook County, against Elk Grove Village and five unnamed Elk Grove Village police officers.
"A call to the police for help should never, ever be a death sentence," family attorney Antonio Romanucci said.
With dozens of supporters looking on, Murray's family and their attorneys announced their legal action at a Thursday news conference.
"Our civil complaint painstakingly details officers' use of excessive force on Jack, a known emotionally distressed person, and their failure to employ effective deescalation tactics," attorney Javier Rodriguez Jr. said.
The incident happened the evening of Dec. 1 on Fern Drive in the northwest suburb, just a few doors down from where Murray's family lives.
The family later learned that Murray had called police to report someone with a knife, and then hung up.
Moments later, officers arrived.
Despite several commands to stop, police officials say Murray continued to walk toward police.
They said a Taser was used twice before gunshots could be heard.
Murray was shot four times, and later died at an area hospital.
Village officials released portions of a 911 call and police bodycam video from the incident.
Murray's family said he was a Type 1 diabetic since age 14, and was suffering from the physical and mental impacts of the disease.
They said Elk Grove Village police officers had responded and helped Murray when he was in emotional distress before.
"He only always cried out for help. It always happened. What went wrong this day?" Donna Murray said.
In response to the filing of the lawsuit, Elk Grove Village Mayor Craig Johnson wrote in part, "The Village once again expresses its sincere condolences to the Murray family for its loss. The officers who responded to this incident all have received combination of crisis intervention training, mental health training and de-escalation training, all of which were utilized in response to the incident involving Mr. Murray."
The incident is still under investigation by the major case assistance team.
Their investigation could take weeks, or even months, to complete.
The police officers involved in the incident had been placed on administrative leave.