In the natural, our consciences are seared as with a hot iron by sin. There is no virtue in following the conscience that is not governed by the presence of Holy Spirit in that life.So, there is in all of us a sense of justice, and we don't want to see anyone who does wicked things like murder, be left unpunished. But then there are the people who do live content lives and are kind and loving and follow their conscience, but simply don't know the Gospel, either because they have never heard it, or because it was not taught to them correctly and so there just in limbo.
One of the key characteristics of a mortal sin in Catholic and Orthodox tradition is Full Knowledge and Deliberate Consent. So, yeah if someone did a really heinous thing that they knew was wrong, and they knew there would be eternal consequence in Hell for, I could accept that, but that's not what you believe. You believe even if they don't know, they are still damned.
God's word tells us that He makes provision for those who have never heard the way of salvation---Jesus.