THEORY ONLY!!!
Natalie has some type mental issue whether sociopath, personality disorder, something going on....not legally insane, but thinks like a murderer. Her reasons for murder could be a thrill kill. Doesn't have to be revenge, or self defense, or whatever. Kinda like serial killers who just kill, reasons that only make sense to themselves. Now suppose this feeling, and thoughts of wanting to murder have been under the surface for awhile. She may have followed cases on here. Who knows. At any rate, the urge is becoming to much to stop. She MUST KILL! She wants to know the feeling of a kill, etc.
She talks to David who may automatically be thrilled, or may have some hesitation. But together they become diabolical. Two evil forces working together to plan their fantasy. Even if David wanted to back out, he's kinda screwed. Natalie could easily hold it over his head that HE Is the one having a relationship with a 13yr old, and HE is the one that is going down if Nicole lets that information be known. Maybe even makes him believe Nicole is going to expose him. Shows him some texts that are forged but he thinks are real. Either way, he's backed into a corner. If his present activities are known by police, the college, and his family, his life as he knows it is over.
Natalie convinces him that she's always wanted to be involved in a homocide, and finds the gore involved fascinating. She may wonder if she can go through with it, thus as backup has David convinced Nicole must be killed if he has any hopes of saving his future, and if he doesn't want to go along, fine, Natalie will just let the police know what he's up to.
David brings Nicole to Natalie as a sacrifice. He may or may not have actually killed Nicole. At this point, doesn't matter. Nicole is deceased. Both involved in planning and coverup. Who actually stabbed her is kinda a mute point. If one goes down, they both will. But secretly Natalie has always had her back up plan, while she didn't think police would EVER connect her to anything with Nicole, and thus she could get away with it, she also knows that whoever speaks first typically gets a 'deal' and less time or, becomes state's witness and gets no time.
So when police start asking HER questions, she knows something has lead them to HER! Not to David, but HER! So she spills the beans, omitting the worse of what she has done in this planned killing, placing the blame on David. Perhaps even gave them the motive that Nicole was going to tell, so David decided to murder her. When in reality, David just didn't want to be busted for being with an underage kid, but murder wasn't his idea!!! For someone planning a murder, none of his tracks were covered. HIS vehicle would be seen on videos, HIS fingerprints possibly on window sill at Nicole's apartment, HIS digital footprints connecting him to Nicole, HIS phone pinging at Nicole's apartment at the same time as Nicole's message to him..... Natalie thinking there's nothing to tie it to her because police will have no need to look at her!
Natalie's phone is in police possession. Nicole's is missing, and I believe David's is missing! Wouldn't be surprised if David had a laptop, that it's missing as well. David didn't keep the Minion blanket!! David didn't have the cleaning supplies. David didn't have the murder weapon. A couple seen buying a shovel? Well Spring is coming, not unusual. And depending on type of shovel, could have been a snow shovel, again, wouldn't draw attention. Later at a another store, a couple buying cleaning supplies wouldn't raise suspicion.
I truly believe Natalie felt this would NEVER come back to her! She'd never be questioned about anything. All evidence was directed towards David.
NOW TO FACTS
The murder weapon was located per the bond hearing today. Not sure where. But even if in the pond, could hold fingerprints. Oils form fingerprints, water alone may not wash them off. Where it was purchased, whether it was a kitchen knife missing from a set, a hunting knife, etc will also give clues to who owned it. Putting that knife in a perp's hand is going to be harder. Humans want a motive. Exposing underage sex could be motive. However, thrill kill can be a motive as well.
JMO!!!