Note: at Constantine's time, the doctrine of forgiveness was that it was not possible to forgive mortal sin after Baptism (this held even longer in North Africa), but was gradually being subsumed by the Church doctrine of having the authority to bind and loose, which meant that there could be formal absolution of sin by authorized Church clergy. As it was in dispute, Constantine took the safe road. This is not to justify him, he did many awful things. What he did not do was blaspheme the Holy Ghost, which remains the only unforgiveable sin to this day (the reason being if you do not believe that the Holy Ghost inheres in Holy Mother Church, then you can't believe that you were absolved, so you can't be absolved). So, when he was baptized, all his sins were indeed washed away and he died in a state of grace.
It was not the bit hypocrisy that I infer your post suggests.
In Christ,
Deacon Paul+
Why then are you writing a piece to justify him?
If Constantine's "conversion" was genuine, why did he put off Baptism? So that he could still get away with mortal sin as a supposed Christian? That's what you imply, in which case one has to conclude that he was not a repentant Christian at all--and you cannot separate Christian faith from true repentence, turning away from one's sin.