Eren's already pivoting backwards, wrenching the knife out of Karkat's shoulder. He's not fazed by Terezi's teeth - he's looked way more fierce monster's in the mouth, her teeth might be sharp but they aren't in any way threatening. He is curious about these weird grey people. He met one, once, and he was fucking weird, but...he seemed harmless.

He points the knife at her, moving backwards and hoping there's nobody directly behind him. "He's the one who jumped at me</>!"

There's no point or reason to defend himself and his actions in a fight to the death, but...But he's not a monster, and he doesn't want to do this.