This was the man who was behind the spread of Christianity throughout the Empire. He claimed that Jesus made him an apostle by appearing to him. He told the Corinthian Christians who had fallen away from belief in the resurrection of Christ that Jesus rose for all the dead would be lost if he did not. He was trying to emotionally blackmail them to believe. That was a very very cynical and callous lie for he was trying to make those who doubted the resurrection feel so scared and depressed that they would believe.  He was plainly lying for if Jesus was a fraud there could be another saviour who is not. He cannot be trusted for Jesus being a fraud could not mean that the dead would have to necessarily be lost. He clearly was hard up for evidence for the resurrection. He knew that the stranger a claim is the more evidence it needs. He was trying to make people dishonest with themselves and believe such a serious claim with such flimsy evidence.
That the apostles tolerated this man and even gave him the hand of fellowship shows that they were just as bad.