The fact is even before you could think to form an opinion in your conscience you already know what is right or what is wrong. Conscience is your very personal oracle, your inner being with a voice, always on the stand by, it is your human sense of right and wrong, or if it is bad, just good or better, of what is sensible, reasonable and adequate, how much is enough and how far is far enough?
If you ignore the voice of your conscience, with an attempt to outsmart and find some short cuts, then just as you do that you put yourself under the effect or influence of some authority beyond your being. It can be lust, greed, or fear of something or someone. Sometimes religious fanaticism can impair your judgement, by superseding your conscience. Strong feelings of attachment, within a nation or family, too can make people become prejudiced, biased and discriminatory.
I cannot say acting against the voice of your conscience is always evil, as sometimes even your conscience needs moral support and encouragement to become clear and vociferous enough. Good influences, good company, education, religious as well as secular, can make us more conscientious. But eventually we need to come to be ourselves, and for that, we need a healthy and strong conscience to live by.