(NBC News)— Tonight, convicted murderer Drew Peterson continues to maintain his innocence in the death of his third wife, Kathleen Savio, and the disappearance of his fourth wife Stacy Peterson, telling “Dateline” in his first interview in over ten years that he was a “loving husband.”

Natalie Morales also speaks with Drew’s brother, who is breaking his silence for the first time.

Here’s a preview of the “Dateline NBC” exclusive, “Infamous”:

Very nice. Eager to please. That was Drew Peterson. Except, Sue says, when Mr. Nice guy morphed into Mr. Hyde— using insults to manipulate Kathleen.

SUE DOMAN: He would say that she’s– she’s– crazy. She was lookin’ like a dog. She was ugly. She wasn’t a good mom.

Soon, the cutting words became heated arguments.

Drew’s brother and sister-in-law, Paul and Norma Peterson:

PAUL PETERSON: They were always fightin’ like cats and dogs. And–

NATALIE MORALES: You saw it firsthand.

PAUL PETERSON: Yeah. We saw it firsthand. they were always mad at each other and it was always something.

The end of the 9-year marriage came one day in 2001. It came in the mail.

SUE DOMAN: She called me and she said, “I got a letter, and it said– he was havin’ an affair, and she was a laughingstock.”

The anonymous letter included this shocking tidbit: Drew’s “other woman” was still in her teens.

SUE DOMAN: the– thing that really stood out with Kathleen was that she was a baby. She said, “I don’t understand why he would do this.”

NATALIE MORALES: Come to find out that was–

SUE DOMAN: Right.

NATALIE MORALES: –Stacy.

SUE DOMAN: Right. Right.

Stacy— the future Mrs. Peterson.

Kathleen demanded a divorce, half their investment in the house and half of Drew’s pension. He OK’d the divorce— not the terms. Kathleen stood firm.

NORMA: She was a fighter. She wan– you know?

PAUL PETERSON: Right. Wasn’t gonna give up.

NORMA PETERSON: She wasn’t gonna give up. She wasn’t gonna let him win. She felt like she had the truth on her side

Tonight hear from Drew Peterson, only on “Dateline,” at 9 p.m. on WDTN.