something that i notice is often overlooked in older films, and even in current ones.
Mad scientists, to build even half of the crap that they build, have to have an amazing grasp of skills and abilities.
I mean, Frankenstein created life from a mismatched set of deceased body parts!
Rotwang built a sentient Automaton and the means to forge realistic flesh over it!
Professor Fate, in the 1890's-19-oughts, built one of the most advanced cars in the world in his own workshop-by hand!
death rays, shrink rays, rockets, doom lasers, Robots, mind-control machines: Mad Scientists have to be some of the smartest people in their respective worlds! no wonder they're so dangerous!
Of course, sometimes they have no idea if it will work.
It's geniuses who use their powers for evil instead of good who become villains D:
Technically speaking though, most are not mad scientists, but mad inventors. Never do they hypothesize "I predict that my death ray will annihilate Manhattan, and I will use Brooklyn as a control group." No, they just get giddy with their toys...
honestly, what i find more impressive is that lone guys working in caves or old castles and entirely reliant on their own resources are able to do by themselves what is cannot be done by think-tanks of equally skilled scientists who have full government backing.