This is ANOTHER dark/angsty au that I imagined in school.

I quickly made a comic out of it in class, I will write the words since I have a quite messy handwriting-

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"You're dead..."

"Found you...!~"

"Found you!"
"Ah!"
"You scared me, Kacchan!"

"Geese Deku is really bad at hiding!"
"Ah...yeah..."
"Wanna know why Deku is really bad at hiding?"
"W-Why?"
"Because Deku is Deku!"
This is not the Kacchan I know...
No,
This isn't even Kacchan!-

Looks
Like
Deku
Was
A
Crybaby
Until
The
End,
Huh
.
.
?

Was all of that really a dream?
It felt too real to be a dream...

"Yo, clean up my mess, would ya?"

————————————————————————

So basically, in this Au, Katsuki does not change his personality after his quirk manifests,
No,
He quite literally changes as a person.

Unbeknownst to Izuku, Katsuki was basically possessed by a bloodthirsty demon the entire time who only wanted to end lives, and he was forced into helping the demon out, yet he lost all of his memories about what had happened.

The only reason why the demon didn't kill Izuku after he literally witnessed him murdering a fellow classmate that nobody remembers- (Not even their parents-) is because Katsuki didn't want him to die, he tried to fight back the demon, but he utterly failed, and the demon wondered why Katsuki didn't want him to die.

Back then, Katsuki was Izukus best friend, so it's no wonder he wanted to protect him.

Izuku completely forgot about everything; the victims, dates of death ,etc. So when he slowly starts remembering, he completely freaks out.
.............................................................................

If this idea interests you, feel free to make a fanfic out of it!