My understanding of the law is that sexual assault needs true sexual contact, Different states may define it different ways, but I think just engaging in explicit talk would support a Corrupting a Minor, or a Child Endangerment type charge, but not Sexual Assault. Likewise, receiving sexually explicit photographs of a 13 year old may technically constitute Possession of Child *advertiser censored* type charges, but I think most proscecutors would handle it as Child Endangerment unless there were extenuating circumstances such as the recipient being a documented sexual predator.
As for motive, as horrible as it seems, the primary motive might well be "thrill kill", perhaps under a thin veneer of "She might turn me in". My guess, and it is only a guess is that DE and NK "fed off each other" and awakened / developed latent fantasies in each other regarding the thrill killing of another person. As individuals, they may, or may not have been capable of such a crime. But, using each other as reinforcement and encouragement, they became capable when acting together. Then factor in that two violence inclined people can function as a miniature mob where it is psychologically easier to minimize individual actions. For example, I did not loot the store, I just broke the window. In the case of this crime, the line of thought might be: I did not actually kill the victim. I just facilitated the preperations and assisted later.
