Why could the Pharisees have put Paul to death?
The Pharisees were enraged because Paul declared that no one is righteous, not even the most religious Jew. They saw themselves as righteous in their own eyes, so his teaching that there is no difference between Jew and Gentile struck at their self-righteousness and provoked them to the point of death.
Les teaches that the Jewish people had for almost 2000 years been told they were special, a covenant people set apart from the rest of the world, not to intermarry. When Paul said Gentiles were just as good as Jews, it sent them into orbit. They followed him from city to city, trying to defeat him in any way, and plagued him until his death. The crowning reason was his declaration that there is no difference between Jew and Gentile. When the Pharisees heard that none is righteous, no not one, they could have put him to death, because they were righteous in their own eyes and Paul made it plain that no one is righteous, not even the most religious Jew.