Lewis just wanted a victim, it didn't matter which agent it was, any female would do fine. This was most likely his idea of how to commit the perfect crime and make someone vanish without a trace. He could have found out anytime during their meeting or after he abducted her that she was a Broker. It was most likely printed right on her business card.
Initially, I thought he targeted her.....but after all of the referral information came to light, it appears that it was most likely random.
I still think he dug the shallow grave in advance of the abduction though, that is really the only way it could have happened and still be in the timeline his neighbor described. When he got his truck stuck in the mud earlier that day, and called his neighbor to help pull him out, he wasn't worried about witnesses at that point because no crime had been committed yet.
His neighbor said she saw him washing the mud off his truck later that afternoon, he had a scratched up face, wasn't smiling, and had shaved his head.........all after around 5:30 p.m. when he would have met Beverly. I assume she said "afternoon" because it was still daylight. That is a very, very small window of time for him to have done much other than grab her, speed over to the burial site, cover it up, and drive home. Only Lewis himself would know what the motive was, and I have doubts it was even a sexual motive, there just wasn't enough time involved.
I think this is one of those bizarre crimes where the perp decides to kill somebody simply to see what it feels like. With all of the weight loss his neighbors described, I have no doubt he was speeding at 100 mph on crystal meth and God only knows what else. He is just chock full of self-loathing and rage, and coupled with the drugs, it gave him the imagined courage to try and pull off what he considered to be the perfect crime.