The Purloined Letter by EA Poe. Hidden in plain sight, the police couldn't find it, they had only looked in places they would have chosen. If you have 2 different storage rooms, or closets, fasten one with a super hefty lock and hide your item in the unlocked place.
Too clever by half

. If I were a murderer who went on frequent long runs at night, I'd leave my phone at home one night, take a new route, head into some
woods*, and bury the thing.
* Oop. Pullman is not in a woodsy area. It's in the Palouse, a huge area of rolling hills and agricultural development. That said, there are heavily treed parks and things in the town, so that's one option, but there are also thousands of acres of unoccupied land around the town, and paved trails and a scenic byway of a couple hundred miles nearby. Would not be difficult to dispose of a small object.
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I'd also immediately smash any smashable (wooden) elements, separate handle from blade if possible, and deform/break the blade as much as possible, scattering small bits in all different places, ideally underground. I don't possess a criminal mind, but I do have a brain, and I think that if I'd gone to the trouble to plan something so big, I'd have planned some ways to distance myself from anything I wore or possessed at the time/place of the crime.
That makes sense. Thanks for explaining.

I would also think it would be pretty tough to convince a judge that he's sane enough to pass a doctor's level semester in Criminology but too crazy to be held accountable for killing someone.
An insanity defense isn't an option in Idaho (Idaho abolished in in 1982, and is one if justv4 states to disallow it; one would think a criminology student would know/consider this and specifically not choose one of those states, assuming he doesn't have a death wish, but I digress.... There is a possibility of a "guilty but insane" outcome in ID, but that is still a guilty verdict. And notably, in ID a GBI verdict does not absolutely preclude a death penalty. Of course, charging/ prosecuting/sentencing decisions may vary, but technically speaking, being deemed "insane" doesn't mean you can't potentially face the harshest punishment.
[And in general, insanity defenses are far more rare than public familiarity with the concept would suggest, and exponentially rarer than that as a successful defense, even in states where the defense exists.]
Proving incompetence to stand trial (a totally different concept and standard) is a very difficult task - and it is only temporary, until a person is stable enough to understand what is going on.