Ted Light told an acquaintance that he had acted on ''the killer instinct that all humans have'' when he fatally shot a homeless man in the chest with an arrow fired from a crossbow, the acquaintance testified in court Monday.

''He said he had been shooting at telephone poles and he had been shooting through sheet metal when he got this urge to kill,'' said Daniel McCarthy, 19, who testified about how Light confided to him that he had killed a man.

McCarthy, who is in the Air Force, testified that he asked Light why he had killed the man and Light blamed the urge on ''the killer instinct that all humans have.''

Light, 21, is on trial before Cook County Circuit Court Judge Earl Strayhorn for allegedly killing Gaylord Tolbert, 33, who had been sleeping on a cardboard box on a loading platform on Lower Wacker Drive under the Executive House hotel on Sept. 17, 1987.

Tolbert had been sleeping in a place frequented by homeless people when an arrow with a pyramid-shaped, serrated-razor point pierced his chest, according to testimony. His killer fled and left Tolbert to bleed to death.

Light was arrested two years later after he had bragged about the killing to many acquaintances and drove a carload of friends to the scene to show them how it had been the perfect crime, authorities said.

Prosecutors Anthony Calabrese and Rimas Cernius told Strayhorn that none of the friends went to police to help in the investigation. An anonymous caller turned in Light and his friend, Dewight Lambert, 30, as Tolbert`s killers, Calabrese said.

Light`s defense attorney, Gary Sternberg, told Strayhorn that the friends did not go to the police with the information because they didn`t believe him. ''He had a reputation for telling tall tales, for telling stories,''

Sternberg said. ''He tried to take credit for things he just didn`t do. That is the reason no one believed him.''

Sternberg also told Strayhorn to view testimony from Lambert, the key prosecution witness, with caution because Lambert, a convicted burglar, had reasons to lie.

Lambert, a Northwest Side resident, testified that he and Light had been friends for years when he showed Light his crossbow and arrows. Lambert testified that Light bought a large crossbow and ordered hunting tips, the serrated-razor heads, for the arrows.

On Sept. 16, 1987, Lambert testified that he drove Light to a park where they saw a stray dog that Light wanted to kill. When they got out of the car, they could not find the dog, he said.

Upset, Light asked Lambert to take him to Lower Wacker Drive where he could find some large rats to kill, Lambert said. But downtown, Light could not find any rats, either, he said.

As Lambert drove away, Light told him to stop.

''He picked the bow up and pointed the bow out of the window and shot it,'' Lambert said. ''I seen a man that was sitting up-he fell back down. I drove very fast.''

Lambert originally had been charged with the murder but the charges were dropped because police did not have enough evidence to tie him to the killing. Lambert said that he received no preferential treatment in exchange for his testimony.

Peter Velic, 21, another friend of Light`s, testified that shortly after Tolbert had been killed, Light had bragged about the shooting.

A few weeks later, Velic said, Light drove a carload of friends to the scene and showed them the cardboard box where Tolbert had lain.

''He said it was like a perfect crime,'' Velic said. ''There was no one around at the time.''