Why did Luke include Jesus forgiving the thief on the cross?

A:The earliest gospel, now known as Mark's Gospel, mentions the two thieves crucified with Jesus but does not say whether either of them repented. Since we now know that the Gospels of Matthew and Luke were actually derived from Mark, both authors were free to interpret this silence as they chose. The author of Matthew's Gospel decided that they both mocked Jesus and neither of them repented, while the author of Luke's Gospel decided that only one of them mocked him and the other repented and was forgiven by Jesus.

Luke's Gospel says that one of the thieves mocked Jesus, but the other thief rebuked him and repented for his sins, creating the occasion for Jesus to talk about seeing him in paradise (Luke 23:40-43): "But the other answering rebuked him, saying, Dost not thou fear God, seeing thou art in the same condemnation? And we indeed justly; for we receive the due reward of our deeds: but this man hath done nothing amiss. And he said unto Jesus, Lord, remember me when thou comest into thy kingdom. And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise." No one has ever suggested that Luke was present at the crucifixion, to give a more accurate account than Mark. He simply elaborated on the original because this suited his theology.