12 Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother’s were righteous.

Read 1 John 3:12 Using Other Translations

Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.
We should not be like Cain, who was of the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother's righteous.
We must not be like Cain, who belonged to the evil one and killed his brother. And why did he kill him? Because Cain had been doing what was evil, and his brother had been doing what was righteous.