Spring-Lock Failure And Death: An Analysis
Quick Disclaimer: Lotta descriptions of gore in here. Also I have like a single semester in Anatomy, I'm not an expert, this is more of a writing drabble for my own reference if anything. But Scott Cawthon probably has done less research than me so I’m inclined to speak my truth.
ALSO BECAUSE I'M PETTY (¬_¬ ) When I say "spring-lock failure", the spring-locks themselves are not the cause of death. The cause of death is getting a robot endoskelton, previously held back with spring-locks and gravity, embedded in your flesh. Spring-locks are just the locking mechanism. This mix-up has bothered me for years.
*this was written before the FNaF movie btw. Been in draft since January 25th, 2021. It’s a fun time capsule*
The problems being violently crushed by robot parts cause:
To start, lets discuss bones. Considering the force it takes the crack bone is about 4000 newton's of force, at least the weight of of five standard pickup trucks, its unlikely any bones were actually broken within the process of the spring-lock failure.
However, that is only when supporting weight, and the suit pieces were unwound so rapidly in a way that they aimed for specific pieces in the bone, therefore it would do more harm because it might do more damage to one specific spot, and gravity working in tandem might be able to crush the rest of them. Still, that would only be enough to crack or damage the bone, so most would still be intact.
Depending on how many individual springlocks holding back individual suit pieces there were, it is very possible the combined amount of damage to the bones resulted in multiple fractures. If not, the metal bits could have embedded themselves into his bones, as bones are quite dense, and the sudden force might've gotten them stuck, but not enough to break the bone as a whole. Hence why Springtrap's got those weird little metal things embedded in his body n stuff.
(Yes. Yes I know about the bite of ‘83. For the sake of this discussion we are assuming that was an astronomical failure on a small child with brittle bones, Crying Child Georg is an outlier adn should not be counted. Also I don’t think his skull was entirely crushed but thats a different subject.)
Okay, but how did it kill him?:
Regardless of whether Willy actually got crushed like a grape in a juice press, he would've died of severe blood loss and multiple vital organs being punctured, and would've gone into like four different kinds of shock. Severe blood loss takes about five minutes to drain most of your blood to a deadly level, and that's if he's lucky, seeing as it's more likely his lungs and heart were some of the first thing to be punctured.
The lungs and heart are delicate, and those being damaged would've cause him to go into circulatory shock, which drains your blood faster and causes there to be a loss of oxygen, which could've caused serious brain damage had he actually managed to survive. The heart can take a bit of damage after being punctured, but the blood would've pooled around it and negate it's ability to pump blood. The lungs being punctured would've caused the air being held to flood the area around the lung, causing them to collapse and make breathing painful and increasing difficult. And either of these things will lead to next issue: Oxygen towards the brain.
Studies show the brain can only go four minutes without oxygen, but that's in controlled medical environment, so it's likely the brain would go much faster, especially if the endo parts managed to puncture the skull. Most farm animals take ten seconds to complete brain death.
It would've been excruciatingly painful for about the two seconds where he was still conscious, but couldn't have lasted more than a minute or two, and that's being generous. It is only through ghost shenanigans and pure annoying willpower his pain is prolonged.
TL;DR: William probably immediately passed out after that cutscene faded to black in FNaF 3 and then died of blood loss. On the plus side, those like three seconds he was still conscious would've been filled with excruciating pain, and depending on how much you want him to suffer and if you pull supernatural shenanigans, you can headcanon that period of time as long as you want.