This psalm gives far reaching promises to which few passages of the Bible compare.
While people will fall dead all around the righteous, it will not happen to God's people (vs. 7). "No evil will befall you, nor will any plague come near you (vs. 10). God's angel will not even let us strick our foot against a stone (vs. 12).

Do these words mean that someone who suffers any of the difficulties described has been unfaithful to God? That seems to be the way that Eliphaz uses these kind of pictures when he presents the picture of the righteous in Job 5:17-27. He uses it to accuse Job of sin.

Satan quoted these words to Jesus in Matt. 4:6 believing the words exempted Jesus from suffering. I think the use of this passage in Matt. 4:6 shows that we must interpret Psalm 91:11-12 through the crucifixion and resurrection of Christ. The words of Psalm 91 will be fulfilled but we may not see a full working out of this in this life. If we do not see them worked out in this life they will be fullfilled in the life to come in a deeper and richer way than we can imagine.