While reading through all the discussion about motive, and NL's level of involvement, I remembered something. Very early on, the rumor was that a Marine had been off base during the time Erin went missing, and his wife did not know where he was. He comes back home like nothing is wrong but Erin is never seen again.
I suspect that scenario is more or less true. I also think CL had very little feeling, if any at all, for Erin. She was really just a notch on his belt. Then she got pregnant and he freaks out. He probably was afraid that NL, who seems to be a pretty tough cookie, would keep his daughter from him, or at least poison her against him. I suspect she takes not prisoners. Even worse, he will owe Erin child support for the next 18 years. And if NL divorces him, he'll owe HER child support too. So, he plays Erin for time while he figures out what to do. Then he lures Erin out to the desert and kills her, as he sees it, to keep her from wrecking his life.
The next morning, it's all over base that Erin is missing. NL, apparently not being a moron, realized CL was gone all day. She puts two and two together and gets a big fat four. She confronts him, gets the truth out of him and then swings into action, trying to save her family. So, here she is, doing everything she can to save them all, and CL can't even keep his stories straight when the police question him. And I think that is what prompted her to explode at him, about how stupid he was, in front of IM.
So, based on that, I think the motives were money and love of his daughter. Mostly money.