Genèse 4:8

And Kain said to Habel his brother, Come, and let us two go forth into the field. And it was that when they two had gone forth into the field, Kain answered and said to Habel, I perceive that the world was created in goodness, but it is not governed (or conducted) according to the fruit of good works, for there is respect to persons in judgment; therefore it is that thy offering was accepted, and mine not accepted with good will. Habel answered and said to Kain, In goodness was the world created, and according to the fruit of good works is it governed; and there is no respect of persons in judgment; but because the fruits of my works were better than thine, my oblation, before thine, hath been accepted with good will. Kain answered and said to Habel, There is neither judgment nor Judge, nor another world; nor will good reward be given to the righteous, nor vengeance be taken of the wicked. And Habel answered and said to Kain, There is a judgment, and there is a Judge; and there is another world, and a good reward given to the righteous, and vengeance taken of the wicked. And because of these words they had contention upon the face of the field; and Kain arose against Habel his brother, and drave a stone into his forehead, and killed him.

CAIN SAID "..." - It seems to me that [Cain] said to [Abel] all of the ways that God had rebuked him. One must have been born on a dismal day to ask the question, "How did he kill him without [the advent of] a sword?" What an astonishing question, since he could have killed him by throttling him with his bare hands, moreover, there were thousands of sticks and rocks about.

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.

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

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

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

ויאמר קין אל הבל אחיו, apparently, Kayin had no remorse, did not try to respond to G’d’s promise and warning. Proof is that he proceeded to do something far worse than what he had been guilty of thus far, by killing his brother.

ויאמר קין, Kayin accused his brother of being the cause that G’d had rebuked him. As a result his anger had intensified. He did not attack Hevel physically at that time as he was afraid of the reaction of his father. He kept his fury under control until the two of them were alone together in the field. This is when he killed him out of jealousy.

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.