Could just be the guy was off his nut and wrote what he thought were the secrets of the universe (told to him by psychic aliens) in what he thought was a perfectly valid code, but actually it's literally the scribblings of a madman. We have any clues to his mental health etc other than that he was obsessed with codes.
See "A Beautiful Mind", ever? Connections where there aren't any.
As many have said, given the sheer amount of potential cyphers, and the existence of one-time pads some of which are - literally - based off natural random number generators (lava lamps, radioactive decay, cloud patterns), it's not even possible to judge whether it IS random bullshit or not. It could look like white noise and still be valid data once decrypted. Simply deciding whether there may be useful information in there is an NP Hard (or whatever you call it... aka "takes an impossibly long time") problem, before you even get down to the decoding itself.
Hell, pull apart a Zip file in a text editor, or play an MP3 as raw PCM in a wave editor, and tell me that stuff doesn't look/sound like highly amplified AM radio interference. But once subjected to the proper decoding algorithm (not even some kind of pad or cypher, just a bit of computer code) it produces recognisable data. In fact, recognisable data that may be 10x or more larger than the coded version.
(And Enigma looks like so much random mashing of typewriter keys, until you get chance to pull apart a captured machine and see how it works, note the mistakes and inefficiencies with the design that give you a possible "in", and the laziness and various slips committed by the users that provide others... and even then, it required the 1940s equivalent of a folding@home supercomputer and some of the period's finest mathematical minds to figure out a thing that was created by some smalltime german researcher banging a few bits of brass and jumper cable together)
So this could be an Ascii representation of a small zip file, even. One that probably encodes a full size page of plaintext. Problem is when you have this kind of encoding, a single unreadable or misinterpreted character can foul up the entire result. Which is why that sculpture had to have a single-character errata note issued for it.
ANYWAY...
Caligula: Have you told the FBI yet, then? And are you ready to spill on what this note you claim to have cracked in 8 hours actually says, the guy's occupation, what it all means etc?
Lucibear: Uh, fancy explaining that in a little more detail (or, less but more relevant detail) please? I don't follow how you got the address of the mental institute from the first group of letters, or why you decided "N" was the letter with significant ones preceding it, with a pretty short sequence of output characters that could be sort-of connected to the first ones (...providing that nurse was even working there at the time he visited).
And are you in fact posting to us from said facility itself? I'm getting Beautiful Mind vibes off your explanation ;-) which won't be put to rest until you actually reveal these missing links.
I mean, I like your theory, it's plausible etc. Nurse gives potentially psychotic patient a fatal overdose of his medication, whether accidentally or on purpose, and he writes a deranged note fingering her as the culprit before wandering off either to die somewhere that he can be fairly certain either the police or an independent passerby will find him first... or just at random in a stoned haze. But, the proof is pretty iffy.