Which murderer does 1 John 3 say "belonged to the evil one"?

Answer: Cain

Do not be like Cain, who belonged to the evil one and murdered his brother, because his own deeds were evil and his brother's righteous.

“Do not be like Cain, who belonged to the evil one and murdered his brother. And why did Cain slay him? Because his own deeds were evil, while those of his brother were righteous.”— 1 John 3:12 (BSB)