A Brooklyn woman who threw a pair of scissors at her stepfather struck him directly in the heart, killing him, police said.

Lynda Delaney, 26, was arguing with Daniel Ilardi, 55, at about 5:30 p.m.Sunday at their apartment on Shore Parkway in Bensonhurst. As the fight escalated, police said, Ms. Delaney threw the scissors at him, and the blades pierced the right ventricle of Ilardi’s heart. He was taken to Lutheran Hospital, where he died from his injuries at about 3:30 a.m. yesterday.

Even before he died, police arrested Ms. Delaney, whom they initially charged with first-degree assault. As of last night, upgraded charges were pending as officials considered the freakishly on-target throw.

The director of trauma at Lutheran Hospital, David Ford, would not comment directly on Ilardi’s injury, but spoke in general terms about such an assault. “I never personally have been involved in such a case, but I’m sure anything is possible,” Dr. Ford, whose level-one trauma center treats 50,000 patients annually, said.

Dr. Ford, who did not treat Ilardi, said penetrating stab wounds require surgery a third of the time, but wounds to the center of the chest — below the clavicle and above the ribcage — make patients “far more susceptible” to major cardiac or thoracic injury.

Dr. Ford said he could not say how often that zone is breached, although an injury’s potential to become life-threatening is “definitely greater in that region.”

According to police, Ilardi was stabbed once in the chest. As of last night, Ms. Delaney’s actual target for the scissors was unclear.

Sources said the two may have been arguing over their living arrangements, with Ilardi urging his wife’s adult daughter to move out. Ms. Delaney has a page on the networking Web site MySpace.com on which she describes herself as a “gangsta.” She says she works as a security guard and is a single mother of a 2-year-old daughter. “Born and raised in Brooklyn NY. But plan to move out of this craphole called NY,” she wrote on the site.

The murder shocked many of her immigrant neighbors on the residential street in Bensonhurst, where multifamily units overlook the Belt Parkway.

According to one neighbor, Kudihuan Wu, 29, the family kept mostly to themselves, although they threw weekly barbeques and parties that noticeably raised the street’s noise level. “The block was a lot quieter before they moved in,” Ms. Wu said.

Others were upset that such a violent incident took place on a street where neighborhood chatter has revolved around traffic on the highway across the street and a handful of small robberies.

“It’s too sad,” another neighbor, Frank Campanile, said. “Where are we going in this world? Where are the values?”