I'm gonna step outside of my normal haunts on this sub here and lean back on my experience working in the power industry for a while.

Let's get one thing out of the way immediately: 120v AC will absolutely kill you and real people have really died in similar ways including Thomas Merton and Claude François. That's why we have safety regulations and building codes. The National Electrical Code first moved to incorporate Ground Fault Control Interrupters in 1968 -- that's those little switches that you see on sockets that pop out when there's a short. Basically, the GFCI can tell when there's a direct path to ground and it breaks the circuit rather than letting current flow.

Those outlets were first required outside around water -- like near swimming pools -- but have gradually proliferated into other areas in which we might expect water and electricity near each other. Why? Because water can create a quick, but fundamentally mobile path to ground, making anyone in the water part of the circuit.

But back to toasters. Why toasters specifically? The history of electrification is an interesting one but it was lighting more than anything else that drove it. Indeed, the so-called "Edison Socket" -- the one that you probably think of as a "light socket" -- was the ubiquitous power socket of early electrification. There were no wall plugs -- the wall plug as we think of it today wasn't even patented until 1904 -- twenty years after first commercial power plant spun up in the United States.

All of that is to say that there was a time after the invention of the electric light but before the ubiquity of the electric appliance where such appliances were rare and simple. The toaster occupies that intersection -- it is simple (just a wire through which current passes), small, and, arrives early on the scene of electrification.

The first toasters -- made around 1905 -- were little more then an open coil of wire that you screwed into a light socket.

From a safety standpoint, these things are nightmares. The wires are exposed; there's no circuit breaker; there's not even an off switch. I don't know if there are records of deaths or injuries but there were bound to be a lot of them; they were accidents waiting to happen.

But critically, they were attached to a cord and man portable in a way that very few other electrical appliances would be for some time. Washing machines, dryers, and other appliances would follow, but the toaster was small by necessity and early on the scene by virtue of simplicity: an ideal opportunity for an early 20th century suicide.

And, of course, people knew that electricity could be dangerous. Thomas Edison famously invented the electric chair; the first executions with it were more than a decade in the past when the first toasters hit store shelves. Heck, a power company even killed an elephant with AC current as a publicity stunt. (Not a demonstration in the "current wars" as this comment previously indicated. Thanks to u/NetworkLama for the correction).

This then provides the basis for the concept -- motive, means, and opportunity, as it were.

Of course, sense then we've worked hard to prevent it. GFCI outlets, circuit breakers, even short cords on toasters all conspire to make it considerably harder to harm yourself with one on purpose or otherwise.