Christian Lepore, 35, is accused of killing John O’Neil, 62, on May 28, 2016, at the victim’s home in West Greenwich, Rhode Island.
Kent County Superior Court heard Lepore, who denies murder, had been seen “babbling about aliens” in the run up to the killing.
Lepore, described as over six feet tall and weighing more than 21 stone with a scar on his face, worked as an assistant cook at the time at the Whispering Pines Conference Center at the University of Rhode Island.
Stephen Lane, senior cook at the conference centre, told the court he asked Lepore to leave work on the day of the killing due to his bizarre behaviour and repeated talking about an alien cover-up conspiracy theory.
He claimed Lepore said: “They’re here. The government knows it, and they’re covering it up.”
Mr Lane said Lepore left but kept returning to babble about the theory as other cooks tried to prepare for a wedding.
The court heard police later found Lepore at the dead man’s home nearby.
He was naked apart from black socks, and crouched by the victim, who had severe head injuries, with a blood-covered drum nearby.
West Greenwich police officer Samuel Maldonado said Lepore attacked arresting officers.
He said: “There was no doubt in my mind that he was trying to kill me.
“I’ve never encountered anyone in my career as violent as him.”
Police tasered the suspect a number of times and released full cans of pepper spray with little effect, the court heard.
Lepore even attacked a police dog that ripped his face open with its teeth, it was said.
Lepore’s lawyers, Joseph Voccola and Robert D Watt Jr, do not deny the defendant was responsible for the crime, however, they argue he was extremely mentally ill at the time and could not have formed the intent to kill, so is not responsible for murder.
The trial continues.