Who was Judas in the Bible and why did Paul choose to replace Judas?

The apostles walked from the Mount of Olives back to Jerusalem.They went upstairs to the room where they were staying.The people present were Peter, John, James, Andrew, Philip, Thomas, Bartholomew and Matthew.They all joined together in prayer along with the women and the mother of Jesus.(G)

Peter stood up among the believers and said, "Brothers and sisters, the Scripture had to be fulfilled in which the Holy Spirit spoke long ago through David concerning Judas."He was one of our number and shared in our ministry.

Judas bought a field and fell headlong, his body burst open and all his intestines spilled out.Everyone in Jerusalem heard about it and called it the Field of Blood.