A spokesperson for the DuPage County State's Attorney's Office tells us the charge was formally filed Monday afternoon ... after the man Kennedy allegedly attacked at an Illinois motel last week died over the weekend.

We're told Kennedy appeared in court for a bond hearing after the filing ... though the proceedings were continued until Tuesday morning.

Kennedy is now facing up to life behind bars if convicted.

As we previously reported, officials say Kennedy attacked 23-year-old Josiah Kassahun in Oakbrook Terrace, IL on July 27 and then fled the scene.

Details of the incident are still unclear.

Kennedy was eventually located, arrested, and initially booked for several charges, including aggravated battery in a public place and threatening a public official.

But, we're told Kennedy was hit with the murder charge after the Cook County Medical Examiner's Office ruled Kassahun's death on Saturday as a homicide.

We've reached out to Kennedy's reps for comment, but so far, no word back yet.

Kennedy is a big star in the skating world ... he's appeared in the popular "Skate" video games -- and has often boarded with Rob Dyrdek over the years.