Full lightning can burn and stop hearts

If she did not actually fire a deadly attack, she could not get him to give up.
If he knew she was not bluffing, that boy would have to fight.
Even so, she heard him yelling that he would not get out of the way.
Mikoto finally could not stand it any longer and she yelled out.
A great flash seemed to pierce through her tightly closed eyelids. A great roar burst
through her hands covering her ears. That was not a decorative high voltage attack that
had low amperage. She fired a true lightning spear that was just like the real thing.
Amid that lightless and soundless flash, the sound of a direct hit resounded out like a
fireworks factory exploding.

Even so, the boy did not clench his right hand even in the very end.
In the end, that was all there was to it.

Mikoto timidly opened her eyes and saw the boy lying on the ground a few meters away.
He was motionlessly lying face down and thin smoke was floating up from his clothes in
places like incense. Just like how video game consoles grew hot after using them for long
periods of time, objects gained heat referred to as Joule heat when electricity was passed
through them.
The great amount of Joule heat created by the high voltage current had given the boy
light burns in various places.
However, the boy did not writhe around in pain due to the burns.
“Ah...”
Mikoto suddenly realized that it was over.
That time, the boy would not get up again. That had not been a fake attack. The true
high voltage current had likely stopped the boy’s heart.

- Toaru Majutsu no Index Volume 3, Chapter 3 Parts 3-4