If the Jews had no infallibility in Jesus' time, why do we need it, and how did the Pharisees make converts worthy of hell?

The Sadducees and Pharisees did not have infallibility; they were just offshoots of the leadership in Israel that formed themselves, with nothing in the Old Testament establishing them, so by that time the Jews indeed had no infallible authority. But earlier they did. In Exodus, God gave infallible knowledge through the Urim and Thummim, a priestly garment by which the priest could pose questions to God that received a yes-or-no answer; by phrasing questions carefully you could even work your way to why God answered as he did, and you could ask which books belonged to Scripture, so it could even confirm the canon. Later this was called the ephod, which Samuel, David, and Solomon used, but like the Scripture itself, which was found under rubble in Josiah's day after falling into disuse, the ephod fell into disuse too. So by the time of the Sadducees and Pharisees, under Roman rule, they had nothing infallible; they only thought they were infallible and demanded respect. As for making converts worthy of hell, Jesus says in Matthew 23:15 that they compass land and sea to make one proselyte and then make him twofold more a child of hell than themselves. They did it by drilling the Pharisee mentality into an innocent young person: keep the outside of the cup clean, worry about how you appear to others rather than what is inside, pray at the street corners to be seen. That is the kind of hypocrite they were reproducing, all appearance and no real relationship with God, which is why Jesus condemned them to the greater damnation.