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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.

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.