Answer: Nick used Angie to aid his plot to fake his death, frame Libby, and collect the insurance money. It's unclear if he ever married Angie, who became Matty's legal guardian, but he needed her to gain access to the insurance money. He certainly didn't love her, and once he alone controlled the money, he eliminated her, as she was a liability who could have exposed him. I agree with the other answer that it also simplifies the plot by killing off a secondary character. It also shows just how devious, ruthless, and sociopathic Nick is.
raywest
Answer: I don't think they explained it, but most likely for her insurance money which is the same reason Nick faked his death in the first place. But it's also possible her death was faked as well. Looking at it from the prospective of the writer, it seemed it was easier to kill her off or get rid of her somehow instead of her showing up at the end with Nick and there wouldn't be a way for Libby to kill her without facing jail time for it and it wouldn't make sense for Libby to just forgive her and let her go.
Bishop73Question: Given that the crime is murder why is she paroled after only six years?
Answer: To start, this film gets a lot wrong about the judicial system and law (including the whole idea that Libby can freely kill Nick because she's already been convicted of his murder). In the film, they just say she's charged with murder, but never what degree. In Washington State, 2nd degree murder generally carries a sentence of 10-18 years (not including felony-murder). However, Washington State did not offer parole at the time of the film like other states did. To be released, she'd have a hearing in front of the Washington State Clemency and Pardons Board, not a Parole Board. And it's unlikely they'd grant her a release. But in Texas for example, she could possibly get parole after serving at least half her sentence.
Bishop73Question: How do we know that he put her in the casket?
Answer: It's rather obvious it was him. Nick lured Libby to the cemetery. He knocked her out and dragged her into the mausoleum, then paid off the kid who acted as a decoy. Nick was motivated to get rid of her (though it was implausible that he would have left her alive before putting her into the casket). There was no-one else who could or would have done that.
raywest 
Answer: Because there were exceptional and extenuating circumstances and, technically, Libby was never quilty of the crime she was convicted for and had to prove her innocence. Also, she may have had a gun, but it could never be proved that she held Nick at gunpoint, only that she shot him in self defense.
raywest