In Stacy's case, she was very young and undoubtedly naive, so when the murder happened, she was hardly prepared to deal with it. A teenager from a troubled home just isn't going to have the ethical background that someone older and more stable might have. She knew right from wrong, obviously, but she also trusted Drew and I doubt he told her all the particulars.
And how could she be sure the FBI would believe her story, and not Drew's? He was a 30-year police officer, while she was just a young girl. And as it turned out, Drew got away with Kathleen's murder, even though her body was bruised and she "drowned" in an empty bathtub. :doh: So Stacy might have rationalized that if the police in that town were going to let Drew get away with it, there was no one else to turn to. So I think she tried to get what happiness she could, even though the burden of Kathleen's death hung over them.
I'm not saying that is "right" or "good," but people can become mixed up in things and over their heads before they know what is happening. Drew held all the cards because he was a powerful local cop who knew everyone, and it kept paying off for him. They swept Kathleen's death away as an "accident," and Stacy suppressed the whole thing until she realized her own life was in danger, and then she tried to reach out, but it was nearly too late.
If women really could learn from such stories, we wouldn't read about this happening over and over again.

Oprah has been showing women the way out for years, but people are ruled more by fear than logic. It takes a strong person to walk away from an abusive relationship, but Stacy might not have had that sort of strength. Clearly, she paid the ultimate price for waiting so long.
Stacy wasn't an "innocent" victim, and she was trying her best to escape the evil in her life. Drew was watching her every move, and she probably feared for her children, so I think they became her top priority. He might have threatened to do something to them, for all we know. She probably knew other things about Drew that might help this investigation - sadly, that is why he killed her.