Pretty much any weapon designed to kill many people at once, and induce plenty of pain and suffering while they're at it.
Also the weapons designed to kill small numbers of people at once, even if it's done quickly and less painfully.
Bullying, racism, ableism, sexism - although those things weren't really invented, technically. They're just human behavior naturally being at its worst.
Cigarettes.
Crystal meth, and other dangerous drugs. But then again, all drugs, including medicine, are dangerous if used improperly.
Nazism.
Autism $peaks.
PETA
ABA
Commercials (well maybe not *all* commercials. I tend to feel nostalgia for old toy and video game ads).
Computer viruses.
Invasion of privacy
Tide Pods. Then we'd only have to watch stupid, moronic adolescents kill themselves from sucking down cinnamon.
Society's pressure on people to be "normal". Normal is a bad thing, unless it's your body temperature.