In pp6-7 of his review essay of James Dunn's new book, Did the First Christians Worship Jesus?, Larry Hurtado gives a concise and convincing argument for what Saul of Tarsus (later to be Paul) found objectionable in Christian belief and praxis:
More directly to the question about why Paul opposed Jewish believers, nowhere in Paul’s letters or Acts do we find a statement that Paul’s persecution of them was on account of their supposedly lax (or non-Pharisaic) Torah-observance, their association with Gentiles, or their critique of the Temple. Instead, Acts depicts Paul as proceeding against “all who call upon [Jesus’] name” (9:14), as “opposing the name of Jesus” (26:9), and as seeking “to make them blaspheme” (i.e., probably pronouncing a curse upon Jesus, 26:11).It was therefore the way in which the Christians used and revered the name of Jesus that made them offensive to Saul, and it was Saul's encounter with that Jesus on the Damascus road that resulted in his change of heart towards the Christians and his acceptance and consequent advocacy of their beliefs and praxis (to put it mildly).
Moreover, nowhere does Paul say that his conversion was basically a capitulation to accepting a more relaxed Torah-observance, a more negative attitude toward the Temple, or more relaxed associations with Gentiles. Instead, he refers to the cognitive effect as a “revelation of God’s Son” (Gal. 1:15-16), and in Philip. 3:1-11 he contrasts his former Torah-centric life with his present fervent devotion to Jesus. The Damascus Road experience did not convince Paul primarily to approve a relaxed halakha, but to change his view of Jesus. In my view, that also suggests a lot about what he had previously found objectionable in Jewish believers.