Sforno on Genesis 4:8/ספורנו על בראשית ד׳:ח׳

1ויאמר קין אל הבל אחיו, how upset he was, and how he had become downcast on account of his brother.

2ויהי בהיותם בשדה, away from the presence of their parents.

3ויקם קין, Kayin arose without any previous quarrel, similar to Deuteronomy 19,11 וארב לו וקם עליו, “he ambushed him and attacked him.”