Depending on the voltage, you might die immediately from being electrocuted, or a few days later from radiation poisoning. The stories of the people who died from the Therac-25 machine are probably instructive: https://en.wikipedia.org/wiki/Therac-25#Radiation_overexposu...

There's a Russian Physicist that was in an accident where a proton beam went through his head: https://en.wikipedia.org/wiki/Anatoli_Bugorski He lived but with some brain injury. I'm not sure what the differences in effect electrons vs protons and a focused vs diffuse beam would be though.

It already happened.

"...the Therac-20, which could produce X-rays or electrons of up to 20 MeV.

"This dual accelerator concept was applied to the Therac-20 and Therac-25, with the latter being much more compact, versatile, and easy to use. It was also more economical for a hospital to have a dual machine that could apply treatments of electrons and X-rays, instead of two machines. "

https://en.m.wikipedia.org/wiki/Therac-25

This has been posted many times.

https://news.ycombinator.com/item?id=38458448

Don't stick your hands in the particle accelerator beam either:

https://www.youtube.com/watch?v=8UgKki1tCKI

If it happened after a series of humiliations and misfortunes, you would become a supervillain.

Slow, painful death from radiation exposure.

There's some well-documented cases of people getting Real Fucked Up from irradiation systems that weren't maintained. Plainly difficult has two good examples of these that were badly maintained and put people into the ground in lead-lined coffins: https://www.youtube.com/watch?v=8V4q59o_mjU and https://www.youtube.com/watch?v=vDGN_Q_0jWI