How could he be baptised? Christ had not yet died for his sins.
And if baptism is not a requirement, then why did Christ require it? (Great Commission) Why did Peter tell the people in Acts Chapter 2 to be baptized? And why is it required all thought the New Testament?
Not a requirement? You need to read your Bible.
Because :
Romans 10:9King James Version (KJV)
9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
so, you are saying Jesus was lying to the criminal?