1st. Jesus is sovereign God. His Word is law. So, therefore St. Dismas went straight to heaven after he died, according to Christ’s word. HOWEVER, note that St. Dismas didn’t die immediately upon Christ’s word. But continued to suffer even after Christ died. Why? I’ll list the reasons below:
2. Christ does not contradict His own Word. Scripture says:
1 Peter 4:1 Forasmuch then as Christ hath suffered for us in the flesh, arm yourselves likewise with the same mind: for he that hath suffered in the flesh hath ceased from sin;
Therefore, St. Dismas had to expiate his sin before he went to heaven. Jesus knew this, therefore He did not permit St. Dismas to die before he suffered sufficiently upon his cross (John 19:31-33).
3. Scripture also says:
Romans 8:17 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together.
Note that St. Dismas died in union with Christ. Compare him to the other thief who died in disunion with Christ.
4. Scripture continues:
Matthew 10:32Whosoever therefore shall confess me before men, him will I confess also before my Father which is in heaven.
Note that St. Dismas also confessed Jesus before men (Luke 23:39-41).
5. Note also, that St. Dismas did a work of gold (1 Cor 3:10-15). His confession is memorialized in Scripture for eternity and has converted many men to the Faith of our Lord, Jesus Christ (James 5:19-20).
And St. Dismas went straight to heaven, in accordance with all the Teachings of the Catholic Church.