SHE COULD COUNT her success by the number of lives she had saved. He could count his by the number of lives he claimed.

She dealt with the most vulnerable. He dealt with the most reviled.

But the paths of Melissa Ketunuti, a doctor at the Children's Hospital of Philadelphia, and Jason Smith, an exterminator from Bucks County, crossed on Monday when Smith went to Ketunuti's Center City house to help her with a rodent problem.

It was there, police say, in the basement of Ketunuti's home following an argument, that Smith killed the doctor as if her life mattered no more than the bugs and rodents he'd made a living exterminating.

After strangling her, Smith set her hog-tied body ablaze and then went on to another extermination job, according to police.

Now that police say that they've found who killed Ketunuti - a Stanford University graduate and world traveler who moved to Philadelphia to work at CHOP in 2008 - the reasons for her death seem more senseless than ever.

Homicide Capt. James Clark said that an argument led to Ketunuti's killing, but he declined to discuss the nature of it. Some news outlets reported that Smith, 36, believed that Ketunuti, 35, had belittled him in some way.

At some point during their argument, Smith struck Ketunuti, sending her to the floor, police said. He then got on top of her, tied her hands and feet behind her back, strangled her and set her body on fire to hide the evidence, Clark said. There were no signs of sexual assault.

Smith has no major criminal record. The first time he met Ketunuti was when he showed up at her house Monday on Naudain Street near 17th, Clark said.

According to a police source, during interviews with detectives, Smith repeatedly said: "I don't believe it happened."

Clark said Smith seemed to be in "a fog" during his conversations with police. When asked if Smith appeared regretful, Clark said "somewhat."

Smith's statements to police, along with physical evidence, were enough to charge him Thursdaynight. At his closed-circuit TV arraignment, Smith sat grim-faced, clad in a hooded sweatshirt as the magistrate read the list of charges in Ketunuti's killing, including murder, arson, risking catastrophe, possession of an instrument of crime and abuse of a corpse.

Smith was held without bail and scheduled for a preliminary hearing Feb. 13. James A. Funt, who represented Smith at the arraignment, declined to comment.

Smith was arrested about 10 p.m. Wednesday at the house he shared with his girlfriend and her small child in Levittown's Crabtree section, Clark said.

During the arrest, police shot and killed Smith's dog, although Philadelphia police said they have no report of it. A police source said the law-enforcement officer who shot the dog was part of the Homicide Fugitive Task Force.

The few of Smith's neighbors who answered their doors Thursday said they didn't know Smith, other than seeing him come and go in his truck.

"I'd wave to him, just as one neighbor to another, but he never waved back," said Connor Schweikert, 19.

Nothing in Smith's criminal record hinted at violence, including a DUI, some traffic violations and a disorderly conduct charge from last year for some type of obscene language or gesture.

No one answered the door at his plain, two-story house Thursday. A man who answered the door at a relative's home in Newtown declined to comment. Other members of Smith's family did not return phone calls from the Daily News.

A police source said that Smith was working on behalf of a company called David Bilyk Exterminating, although it was unclear whether he was an employee of the business. Clark said that Smith had been subcontracted by the company that Ketunuti had called for help, but he would not identify that company.

No one could be reached at a number listed for Bilyk Exterminating, and no one answered the door at an address listed for the business in Newtown.

Clark said that police were led to Smith by talking with neighbors, following tips and, most importantly, from video surveillance.

Developer Ori Feibush, who owns the OCF Coffee House, at 18th and South streets, around the corner from Ketunuti's house, was able to provide key video footage to police. He spent hours with detectives poring through surveillance video from his coffee shop and, about 3 p.m. Wednesday, they hit gold.

"We had crystal-clear video of him [Smith] walking right past the coffee shop and looking at the cameras," Feibush said. "I was exceptionally uncomfortable until they caught the guy because it looked like anybody who walks by the shop."

Smith was not in a uniform and was wearing a North Face jacket and a hat when walking to Ketunuti's house, but he looked different when he left, Feibush said.

"When he came back he had gloves on, his hat was off and his jacket was in his arms," he said. "Coming back he walks directly under the camera and that's when we realized, 'Wow!' We had this guy on camera."

Feibush said he ran and grabbed homicide detectives who were outside his coffee shop.

"They were in absolute shock as well," he said. "There was the guy, walking in broad daylight with no hat on, not hiding his appearance in front of a very powerful surveillance camera."

Police said that no one was eligible for the $35,000 reward for information leading to a conviction.

Feibush said that "a weird, eerie feeling" has settled over the "exceptionally quiet" neighborhood since the slaying and that he's relieved that Ketunuti's alleged killer has been captured.

"I honestly believe this guy would still be on the loose for a long time, if not forever, without video surveillance," Feibush said.

The grisly crime has shaken residents on the quiet block in the shadow of the former Graduate Hospital. They, too, expressed relief following news of the arrest.

"I didn't expect it to happen so quick considering that they didn't seem to have that much," said Victor Pisani, who lived a few doors down from Ketunuti. "I guess we can be thankful for video cameras. As far as anything else, I'm just relieved."

Pisani, who has lived on the block since 1973, said that he remembered much more dangerous times. He described Ketunuti's slaying as an aberration.

"It's just something that happened, but could happen anywhere," he said.

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