The 74-year-old victim was shopping alone when he was beaten in a vicious, unprovoked attack.
"It was terrible. I could hear it, but I didn't know he was getting beat up until I went to peek around the corner, and he was laying there in a pool of blood," witness Jennifer Thompson said.
Authorities arrested the 47-year-old suspect outside of the store. Detectives said they don't know what led to the murder.
"It appears it was unprovoked. There does not appear to be any conversation or any exchange of any type of communication," said Lt. Eddie Hernandez of the Los Angeles County Sheriff's Department.
The incident happened just before 5 p.m. at the Wal-Mart on Carson Street. Officials said the brutal beating was captured on security cameras.
The suspect grabbed an aluminum bat from the sporting goods section and suddenly went on the attack, investigators said. The victim was hit from behind without any warning.
"It appears that the suspect approaches the victim and uses a baseball bat to start assaulting him, and then once the assault is over, he drops the baseball bat and starts to walk outside the store," said Hernandez.
The victim, whose identity has not been released, was pronounced dead at the scene.
Witnesses described the suspect as homeless and "on edge," but authorities have not confirmed that information. It's unknown if drugs or alcohol was a factor in the murder.