Mortal sin is deadly sin, those freely chosen acts and dispositions that make the reality of God’s grace (i.e., his love and power) dry up in us. God can no longer relate to us because our sins have pushed God out of our lives. Pride, Lust, Anger, Gluttony, Greed, Envy, Sloth, freely chosen and acted on, can ruin our lives. Take gluttony. A 1200 lb. person is not able to live happily, the way God desires us to live. Or the destruction and greed that Bernie Madoff caused. Social sin, like war or corporate malfeasance, can also be mortal, i.e., deadly.
Mortal sin is not easy to commit. The sinner must know and realize the act is grave matter, have full knowledge he or she is committing such sin, and give full consent of the will (CCC #1854). “Mortal sin is a radical possibility of human freedom as is love” (CCC #1861). We do no one a favor by diminishing human capacity to freely choose grave evil and sin, but the judgment of the person must always be left to God.
Venial sin is to freely choose something of lesser matter (like splitting an infinitive). It diminishes the reality of charity in our lives, but doesn’t rupture our relationship with God.
Most important is to realize that “where sin increased grace abounded all the more” (Rom 5:20). God came to call and save sinners.