When we have sin in our life, we are fully separated from the Lord and this is the worst possible thing that could ever happen. The scary thing is the fact that since we can't SEE God, we don't ever really think about being separated from Him. But when we do evil, we separate ourselves from Him.
This passage from Psalms indicates how awful it is to be separated from God. On the other hand, it is so comforting to those who are righteous. None of us want the Lord to turn His face from us - that's why we need to constantly make sure we are living righteous lives. Because in the end when this life is over, if we are separated from God, it will be forever - and there will be nothing we can do about it. There's nothing worse than being separated from God.