I don’t understand why he would have been judged in that way. He simply asked Jesus why He didn’t save Himself if He truly was who He said He was. It seems like a normal question to me.
We are saved through faith, and faith is the impetus for forgiveness and mercy. His question was one showing his lack of faith. Dismas, on the other hand, accepted the power and glory of Christ as the Son of God and Savior, opening the door for mercy and forgiveness.
We are no different than the two hung on the crosses next to Christ…we are, like those two, sinners. But through our faith that God offers us forgiveness, we can obtain the promise of salvation.
The two are also a lot like Judas and Peter. Both were guilty of the same sin. Judas denied the ability and power of Christ to forgive, and Peter arrogantly denied the need for Christ to save him, thinking he could do it all himself.
And in the end, because of faith and lack of faith, the two were judged differently. But we must not confuse judgment with condemnation. Christ, as he told us in the Gospel, does not condemn, he only judges…those condemned, condemn themselves through their lack of faith.