In the first place, no one knows whether this thief was baptized or not. The Bible does not say. However, we know one thing for sure about this thief, he lived before Jesus said, “He that believeth and is baptized shall be saved …” (Mark 16:16).
The thief on the cross had been dead fifty-three days before Peter preached, “Repent and be baptized for the remission of sins …” (Acts 2:38). A man cannot be held responsible for doing something which is not in effect while he lives. It took the death of Christ to bring the new law into being (Heb. 9:16-17).
Just remember that the thief on the cross lived and died under the law of Moses. We, on the other hand, live under the law of Christ (Gal. 6:2) which says, “He that believeth and is baptized shall be saved” (Mark 16:16). So, why try to be saved like the thief?