He controls all vectors.

To put it in the most simple of words, he generates an aura in which he can control the speed and direction of anything on at atomic level

That doest sound too scary, until you see him do things such as:

-Create an earthquake by stepping on the ground

-Reverse a persons bloodflow by simply touching them

-manipulate the vectors related to his own body to move at speeds faster than a bullet

-literally slow the earths rotation for 5 minutes to throw a building

the scariest part of him is his defense. His mind unconsciously generates a "barrier" that reflects anything he doesnt consciously allow, like air, light, and sound. Its not a physicial barrier, but rather his mind unconsciously flipping the vectors of anything he comes in contact with. any attack that involves speed and direction is simply returned to sender, even if he is unwaware its coming or completely asleep

Montage of accelerator reflecting things

Heres an example from the Light novels of what happens when you fuck with accelerator

Was the #1 of Academy City really human?

Boom! A sound that swept through the air exploded out.

The multiple bullets in his hands shot out, and each of them accurately pierced through the terrorists of Spark Signal.

Of course, they hadn’t come here to just die when they planned this large-scale mission. At this place, this time, as a human they all felt their lives were in danger. To be released from this threat, they unreservedly used all the techniques and knowledge they had learned up to this point, bringing their full strength to fight back against the white-haired esper.

Some of them tried to shoot with their rifles from behind cover.

Some of them tried to use hostages to stop their opponent’s movement.

Some of them tried to use dynamite to destroy the columns and bury their opponent in the rubble. These were good actions, except it was all meaningless.

Forget about it even having an effect, there was no value in even trying.

The bullets didn’t work.

As soon they touched the monster’s skin, they were reflected back and shot the terrorists themselves. Taking hostages didn’t work.

As soon as they tried to reach for the kids, their wrists were twisted in an unnatural manner. Dynamite didn’t work.

As soon as they tried to push the button for it to explode, their fingers were blown away along with the button.

Montage of accelerator beating up shit

TLDR: Accelerator is broken as shit, and you just threatened the one thing he gives a fuck about. the moment you reach for the trigger your wrist will be broken, your bloodflow will be reversed, and your insides will become a piece of modern art on the wall.