1 John 3:10-12, 15

By this the children of God and the children of the devil are clearly identified: anyone who does not practice righteousness [who does not seek God’s will in thought, action, and purpose] is not of God, nor is the one who does not [unselfishly] love his [believing] brother. For this is the message which you [believers] have heard from the beginning [of your relationship with Christ], that we should [unselfishly] love and seek the best for one another; and not be like Cain, who was of the evil one and murdered his brother [Abel]. And why did he murder him? Because Cain’s deeds were evil, and his brother’s were righteous.

Everyone who hates (works against) his brother [in Christ] is [at heart] a murderer [by God’s standards]; and you know that no murderer has eternal life abiding in him. [Matt 5:21-23]