This was premeditated murder. It is obvious that more has preceded this anger. John is talking about Cain’s evil deeds (in the plural, 1 John 3:12), even before this history (see Genesis 4:4). Between these two children of Adam and Eve there was a contradiction that led to hatred and that had erupted now: this nothing
boy (the name Abel!) had to get out of the way, especially now that Abel had torn God’s attention away from him (Cain). If God chose Abel, Cain would show what he was worth. Soon God would have no one to pay attention to. He was not discriminated against. It may be his very own brother, but enough is enough. That is how far jealousy (rivalry) can push a person. We must nip sin in the bud (with yourself, see Genesis 4:7, but also in general in the church).
8 Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him.