A bit of a definition:
“Scrupulous people are sincere, but they are ruled more by fear than by love. Their fear is distorted: small things are seen as large, and unfounded worries about guilt torture the individual from morning until night. The compulsions and obsessions of scrupulosity must be recognized for what they are and treated with care.” (Rev. Thomas Dubay, Seeking Spiritual Direction)
In other words, a scrupulous person sees sin where there really is none, and sees small sins as serious sins. Scrupulosity can interfere with what should be a loving, trustful relationship with God, and turn it into a relationship of fear.
Our conscience should be well-formed and balanced: not so lax that we think nothing is sinful, but not so scrupulous that we see sin where there is none.
The devil likes to work on us through fear, causing us to despair of God’s mercy.