The thief died while the OT was still in effect. The command to repent and be water baptized in the name of Jesus pertains specifically to those living after Jesus' death, burial and resurrection.
Scripture reveals the high priority God sets on obedience:
"If ye will fear the Lord, and serve him, and obey his voice, and not rebel against the commandment of the Lord, then shall both ye and also the king that reigneth over you continue following the Lord your God:
But if ye will not obey the voice of the Lord, but rebel against the commandment of the Lord, then shall the hand of the Lord be against you, as it was against your fathers....And Samuel said, Hath the Lord as great delight in burnt offerings and sacrifices, as in obeying the voice of the Lord? Behold, to obey is better than sacrifice, and to hearken than the fat of rams. (1 Sam. 12:14-15; 15:22)