I was particularly thinking of baby monitors during an emergency. It was most important house-hold device I could think of in terms of harm. Maybe turn a freezer off on IoT fridge while people are on vacation then back on just before they return to make meat refreeze or something spoiled. Maybe turn off the power in household with IoT home automation and someone on life support of some kind.

Im only having a few possibilities come to mind that are life-threatening. Most are just annoying or financial drain. If we add painful, maybe make an epileptic's screen on SmartTV blink fast like the attack on the web site. Turn off people's alarm clock enough they get fired and loose health insurance before major operation. Im really having to stretch it here.

Alarm system with remote fire alarm capability. Those things are everywhere and if they don't work people could easily die.

That's a great one. Good catch.

Lights that turn off at night while someone is walking down stairs can result in death if that person is unfamiliar or disoriented enough to tumble.

That's clever. The threat of lighting was nicely illustrated on Christmas Vacation. Whole relationships ruined and stuff.

https://www.youtube.com/watch?v=rp8lwpvQEIM