Yes, I could, but I'd have a 90% chance of hurting myself. Ditto for giving some cancer with radiation, many covert poisons, and similar. I can name a dozen ways I could kill or harm you, but:
1. I'd need to be wacko enough to want to harm you
2. I'd need to be sane enough for a major engineering project
3. I'd need to be competent enough to succeed without hurting myself
The set of individuals with all the traits required is small. It's not zero -- we had Ted Kazinsky -- but very small. Most major crimes are also never solved, and many more people can just take a gun and shoot someone. That's probably even harder to track down since a lot more people have guns than labs and PhDs. Esoteric technologies make for easy investigations.
So in the end, I think state actors have these. They also make good plot lines for a novel or video game (I have about a dozen I came up with in the context of a half-baked creative project).
But for real life individuals, I don't see how they'd become prolific. We'll probably see a crazy someday, but nowhere close to gun homicides. They just don't make much practical sense for an individual.
The top of my “I’m surprised it’s allowed” list is large lithium batteries in airplanes. If phones weren’t already so ubiquitous, they would have been mostly banned.
My guess is there would be a strong selection against impulsive non-experts. That’s on my “No-MacGyver” list with garage door springs and AC unit capacitors .
