Why would Paul want to be ‘accursed from Christ’ for the sake of his ‘kinsmen according to the flesh’?
You apparently refer to Romans 9, where Paul is discussing the advantages God had given to the Israelite nation, and which they had spurned. He says: “That I have great sorrow and continual grief in my heart. For I could wish that I myself were accursed from Christ for my brethren, my countrymen according to the flesh, who are Israelites” (Rom. 9:2-4).
Paul was not passive, bland, or unfeeling. He had a passionate love for his nation, and realized deeply the singular glory of their position. How could they, to whom God had given such tremendous advantages, turn coldly against God’s goodness? To Paul it was inconceivable.
We cannot think for a moment that Paul truly wished himself accursed from Christ, His Gospel, His service, and the great hope of eternal life which he cherished. The phrase is merely a figure of speech to emphasize his deep concern for his brethren. Notice that he does not say that he wished himself accursed from Christ but “I could wish that I myself were accursed from Christ”–an expression of his deep feeling (emphasis mine).
A footnote in the New Catholic Bible points this out: “So great was Saint Paul’s longing for the salvation of his own kinsmen that he would make any possible sacrifice to that end, even to the extent of being separated from Christ, if it were permissible to entertain such a desire.” That these words are merely an emphatic way of declaring his great devotion to his people and that they are not to be taken literally, is evident from what Saint Paul has just said in the closing verses of chapter 8. He was “persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, nor height, nor depth, nor any other created thing, shall be able to separate [him] from the love of God, which is in Christ Jesus our Lord” (Rom. 8:38-39).