FROM WHAT NEIGHBORS could see, Lou and Judith Hartdegen were soulmates. The pair, both in their 70s, often took walks together through their Castor neighborhood or just sat outside their apartment.

So when neighbors learned yesterday that not only had Judith Hartdegen, 74, been brutally beaten to death inside her apartment, but that her 75-year-old husband was police's prime suspect in the slaying, they were in shock.

"There's no way," neighbor Jessica Ruiz, 24, said during a candlelight vigil she and her sister planned outside the couple's apartment last night. "They were best friends. Soulmates - that's what that was. Deep down in my heart, I know for a fact he didn't do it."

But late yesterday police painted a different picture about Hartdegen's grisly slaying, which at first appeared to be a home invasion targeting the well-liked, vulnerable elderly couple in the apartment they shared on Castor Avenue near Knorr Street.

Lou Hartdegen will be charged with his wife's murder, a police spokeswoman confirmed late last night.

"There's no way," repeated a disbelieving Ruiz at the vigil. "If that was the case, I'm completely and utterly shocked."

When police were called to the pair's apartment about 2:30 a.m. yesterday, they found Judy Hartdegen severely beaten and her husband with injuries to his head and neck. Both were taken to Aria Health's Torresdale hospital, where Judith Hartdegen died. Lou Hartdegen remained hospitalized there last night, but it was unclear how he suffered his injuries.

A torn screen on a rear window and broken wood on the deck around the back of the couple's apartment initially led investigators to believe that someone had broken in and beaten both of them. Police later said they believed that Lou Hartdegen was responsible, but did not say what motivated the killing.

By all accounts from neighbors - some of whom described Lou Hartdegen as the "Mayor of Castor Avenue" - the Hartdegens, parents of three grown children, were beloved, longtime fixtures on the bustling Northeast Philadelphia block where they lived.

"He knew everything about everything," said Mike Visco, who owns Guido's Menswear on the block where the couple lived. "Judy followed him everywhere he went. They did everything together."

Ammar Alshamma, who owns Philly Fashion Accessories and Cosmetics on the block, echoed Visco.

"They are always walking together, taking care of all the property here. They are so quiet, so respectful. They didn't harm anyone," Alshamma said.

He described them as "peaceful people," and recalled once giving Judith Hartdegen a ring from his store as a gift.

"They were passing one day, and I said, 'Take one for free,' " he said. "She was very happy."

According to court records, Lou Hartdegen wasn't always peaceful. In 1991 he was sentenced to 11 to 23 months in prison on a conviction of incest but found not guilty of rape, indecent exposure and indecent assault. In 1971 he was found guilty of burglary, larceny and receiving stolen property.

Last night, neighbors who lit candles and signed an "R.I.P. Judith" poster at a vigil for Hartdegen remembered her as a generous woman.

"She would give you the shirt off her back in the dead of winter," Jessica Ruiz said, her eyes filling with tears.

"What happened shouldn't have happened," her sister, Kristin Ruiz, 18, added.

The Ruiz sisters and others who knew the Hartdegens from the neighborhood said they couldn't wrap their minds around the idea that Lou Hartdegen may have killed his wife.

"He would shuffle, like barely lift his feet up," said Jamie Padilla, 29, demonstrating how slowly the man would walk.

"It takes him about an hour to walk around the block," Kristin Ruiz added.

All who knew the man said they could not imagine how he was physically - or mentally - capable of inflicting such harm on the woman they said he had loved for more than five decades.

"That's one thing I can't understand . . . they were like best friends," Kristin Ruiz said. "They spent 60 years together. They were inseparable."