I’ve only seen two of the Saw films, but I’ve seen clips of the “games” used in the others, and apparently the Collector film(s) have the same idea to some extent. I have to ask, how do they come up with these horrible and yet amazing traps? Do they consult engineers? Makes it a bit daunting if you’re a wannabe scriptwriter like me. 
Maybe I should just stick to a slasher-film conceit…
I’ve not seen any of the Saw films but I’ve heard of some medieval torture devices that would make your hair curl and make you wonder how human beings can be that cruel to each other. There’s plenty of real life examples to choose from.
The filmmakers just think back to all the study hall fantasies they had about what they’d do to the jocks who stuffed in their lockers in high school.
Have you MET other humans!?
On the topic at hand, were there a lot of films before Saw (2004) and Final Destination (2000) where they created elaborate Rube Goldberg traps to kill off the characters? Jason, Freddie and Mike Meyers mostly just poked people with sharp objects.
(bolding mine). Yes I am aware of what assholes people can be, but do a search on Medieval torture devices or look up some of the real life serial killers. I’ve not met anyone who took delight in causing as much physical pain as possible, thank god. But it still amazes me how inhumane people can be.
The Dr. Phibes movies come to mind.
The original* Saw* premise - cutting off your own body parts to escape death - was a ripoff of the end of Mad Max (Max handcuffs a guy to a burning car and tells him he probably can’t cut through the cuffs before the flames reach the fuel tank, but he might make it through his ankle).
SyFy ran a set design competition show called “Hot Set”. One week (for Halloween), the competition was a horror movie set. The two designers were challenged to create an original serial killer lair. They were shown body parts as inspiration to incorporate in their set.
Each lead set designer had two partners they brought in, plus a team of constructors provided by the studio (I think). The set designers competing on that show had a broad selection of backgrounds, usually having worked in the industry for some time, doing commercials or cheap low budget films.
Essentially, for this challenge, they were given a scene where a woman is at her date’s house looking for the bathroom, but stubles upon his secret lair. Everything else is up to the set designer to provide.
This could then be incorporated into whatever story they wish to tell.
Basically, how I would think it works typically is someone creates the concept of how they want their story to be executed, then they get people with the swath of construction skills to make it come true. Set designers and builders don’t necessarily have engineering degrees, but rather practical skills in carpentry, mechanical fabrication, etc. Sort of like the Mythbusters special effects team.
For the SyFy example, one woman wanted a spinning turntable with a torture device in the enclosed chamber. She had to have her team figure out how to make that spinning turntable chamber. The other guy devised a spring-loaded gate triggered by the entry of the actress.
Basically, you just have to be able to envision how to be mean and scary and painful and destructive. Then let the builders figure out how to execute what you want.