Conscience …

Each person must follow his conscience, but at the same time a proper formation of the conscience is needed. 

“An upright and true moral conscience is formed by education and by assimilating the Word of God and teaching of the Church”

Compendium of the Catechism of the Catholic Church (# 374).

If the conscience of a murderer or thief says that everything is OK, does it mean that really everything is OK?