Were we all capable of committing horrible sins before we were saved?
Yes, every person before salvation was capable of committing those same horrible sins, because the old Adam is where the human race comes from. The old Adam has free course when external restraints are removed. Yet where sin abounds, God's grace is always greater.
Les teaches that even unbelievers can be inhibited by their surroundings, circumstances, parental influence, or community. But when a person is placed in a totally strange environment, away from home where no one knows them, the old Adam has free course. He illustrates this with soldiers in basic training who did things they never did in their hometown, and with invading armies that pillage and rape without seeming bothered. That is the old Adam. Les explains that every one of us, before we were saved, would have been capable of those same things, because that is where the human race is coming from. Paul is showing that the old Adam abounds with sin, but where sin abounds, the grace of God is always greater. Verse 21 presents two alternatives: as the old Adam reigned like a king over us, even so grace might reign like a king.