I believe DE was a very socially awkward 18 yr old boy. I also think he may have a personality disorder. Some people on websleuths have mentioned how DE seems pompous, arrogant, wooden, awkward, etc. in the interview he did a year ago regarding his running. I know he also attended a very small Christian school for about 9 years (his formative years). I think DE had very little if any, experience with dating and sexual relationships with girls.
Perhaps he was looking online for young teenage girls he could meet up with to practice his dating skills, but the relationship with Nicole evolved into something else. This is where the more sinister, evil aspects come in. He liked controlling her and toying with her. Somewhere I read that he had asked her, "what are your favorite flowers?" and she responded, "Roses". He then told her he would bring her some. He knew exactly what he was doing, telling her what he thought she would like to hear even though he had no serious interest in her as a girlfriend. That was a form of humiliation and he was enjoying it. DE then gets NK involved and together they plan on how to murder this little girl who they both perceived as inferior, worthless, stupid, etc. I think it made DE, and possibly NK also, feel both powerful and intelligent.
Someone on here mentioned that DE was a video gamer and liked games that involved a lot of violence and sexual themes. These are probably things that turned him on, excited him, and made him feel powerful.
I think DE was the mastermind behind this entire crime, and that he saw NK as someone who was needy and insecure, and therefore he could also manipulate and use/abuse to forward his own agenda.
As far as the shovel, someone on here said they bought it the day/night of the murder, but I thought I had read that after their planning at the Cook-out restaurant they then went shopping at Walmart for the shovel. I believe that happened days before the murder.
Prosecutor has his motive written down in black and white from his own words. Also, "thrill killers" are usually no-life losers living in their mothers basements, with limited future prospects. Eisenhauer had everything to live for, and that is exactly why he killed her... to save his own skin, or so he thought, but he overestimated his own intellect in planning out the murder. He was still in the honeymoon phase of a very promising college career, only 1-2 months in when he met her and he was doing extremely well in classes that he expected would be much harder. With regard to NK, I believe she had 3 motives in agreeing to help and possibly irking it on: she wanted Eisenhauer for herself, wanted to please/help him, and she's also mentally unstable as well as depressed. Misery loves company combined with jealousy. Eisenhauer used this to his advantage. Jmo of course.