פּירוש אויף בראשית 4:8

ויאמר קין אל הבל AND CAIN SPAKE TO ABEL — He began an argument, striving and contending with him, to seek a pretext to kill him. There are Midrashic explanations of these words, but this is the plain sense of the text.

ויאמר קין אל הבל אחיו, “Kayin said to his brother Hevel;” seeing that the Torah does not elaborate on the content of Kayin’s words, we must assume that he told his brother about what G–d had said to him. (verse 6-7) He made it plain to his brother that he was very upset that G–d had not seen fit to accept his offering as He had done with Hevel’s offering. Hevel reacted to G–d having criticised Kayin with joy. When Kayin saw that his brother had been glad that he had been criticised by G–d, he arose with the intent of killing him. (second half of this verse). His motive was plain jealousy. Our author cites an additional or different interpretation of the incomplete sentence: “Kayin said to his brother Hevel,” which he had heard in the name of Rabbi Joseph Kara (turn of 11th century, not to be confused with Joseph Karo author of Shulchan Aruch, of Safed who lived during the most of the 16th century). that initially Hevel, having heard from Kayin what G–d had said to him, was afraid of him seeing that his brother was so angry, and remained on guard against him. When Kayin told him that G–d had held out hope for him by promising that He would relate to him with fondness if he changed his ways, Hevel dropped his guard feeling certain that Kayin would indeed change his ways. When Kayin spoke to him in a friendly mien, i.e. ויאמר, he thought that this was proof that Kayin had changed. He therefore dropped his guard with fatal consequences for himself.

AND CAIN SPOKE TO ABEL HIS BROTHER. He began a conversation of argument and contention with him in order to seek a pretext against him and so kill him. This is the language of Rashi. Rabbi Abraham ibn Ezra said that the interpretation that appears most likely to him is that Cain related to Abel all the chastisements with which G-d had reprimanded him [and Cain accused Abel of having brought them upon him].
But in my opinion it is connected with the following words of Scripture: and it came to pass, when they were in the field, meaning that Cain said to Abel, “Let us go forth into the field,” and there he secretly killed him.
It is possible that his intention in killing Abel was that the world be built up from himself for he thought that his father would not have any more children. He also feared that the main building up of the world might be from his brother, [which seemed likely since it was he] whose offering had been favorably accepted.