Let us first familiarize ourself with Paul's passage in Rom. 11:9-10
The author of Psalm 69:23-24, who was quoted by Paul with somewhat different words in Rom. 11:9-10, did not have grace in mind either when he said:
The psalmist, who was probably Jeremiah and not David, as Paul said, called down God's curse on his brutal and ruthless enemies of God.
Paul quotes the same passage to call down God's curse on his own people the Jewish people...for not accepting his conflicting gospel of justification apart from the Law!
Answer for yourself: How could the Jews do otherwise as Moses was being read weekly in the Synagogues? They Jews knew better than to accept a "false gospel." They knew the message from Paul was contradicting Moses, the Prophets, and the writings. They knew the God who was the same yesterday, today, and forever, and Who changes not. It is we who change and in this case attempted to change the over-all message of God in the Jewish Scriptures.
Psalm 69 is a prayer for Gods judgment against His enemies for excessive cruelty toward the Children of God. This psalm gives extensive expression to one of the major themes of the psalms: undeserved suffering. In this case the suffering has come largely from the psalmist's steadfastness in his commitment to the LORD and His commandments. Ironic that they suffered for what Paul wants them to give up and he advocated suffering in the form of God's punishment upon them for not giving up the Torah.
Now pay attention. It is absolutely ironic that Paul would quote from a Psalm to further his agenda of justification apart from the Law and "supposed" justification by accepting his unique identity of Yeshua in place of obedience to the Torah by using a Psalm which is written in heat of passion of one who says that the zeal of God's House, where the Torah is taught, has eaten himself up. I find that rather amazing. Another ironic thing is that he refers to a Psalm where the writer begs God not to turn His face from His servant (the writer of the Psalm), when all along Paul knew that man's iniquities (sin as understood as violation of the Law of God) separates man from God. Yet Paul wants us to believe in faith alone apart from obedience to the mitzvoth and laws of God. How absurd!
Paul calls down God's curse and punishment on the Jews because they won't accept new Pauline doctrines that contradict Moses, the Prophets, as well as Yeshua and his teachings.
We have already looked at David and see that he did not believe in justification apart from the Law. We clearly saw that David responded to his faith in "works" of penitence, repentance, prayer, and confession in order to be justified before God. This we should remember in light of Paul quoting the psalms trying to used David for his authority for his doctrines. Such is foolishness.
So in summary Paul misquotes the Old Testament again and connects the peoples moral failures as equal to their rejecting his teachings ("his gospel") of faith alone without obedience and works. Likewise, he equates Jeremiahs/ or Davids prayer for Gods judgment upon their enemies with those who reject and refuse his unique and different teachings which opposed the only Bible they knew. We today would not be so confused had Rome not put a substitute Torah together in the fourth century (the New Testament) to combat the Jews who already had "their book".
Answer for yourself: Just what kind of man and what agenda could one have that he would desire that the very ones he is sent to with a message from God be cursed because they reject his unique message of salvation which opposed all they every knew and read since the beginning of time?
Answer for yourself: What kind of sour-grapes theology is this we have followed uncritically in our churches?
I would recommend that all read a book by Hiram Maccoby, The Mythmaker: Paul and the Invention of Christianity. They you will know the answer to the last question. Shalom.