To stay the course on the path of truth and Islam, we tend to focus more on learning about what is regarded as ‘right’ and ‘good’. That’s the way it should be because Islam is all about practicing righteousness.
Both Quran and the prophet (Sallellahu Alaihi Wa Sallam)’s teachings emphasize quite clearly that faith in Tawheed has to be backed up by doing good deeds, which is only possible when we know more about the good and righteousness.
However, living this life makes it equally important to know about the bad, the evil, and the haram as well. Not knowing the difference between halal and haram, and good deeds and sinful behavior can make us unknowingly stray away toward failure. Let us face it: How many of us indulge in sinful behavior and yet do not think of it as such? How many more knowingly commit sins simply because we are unaware of its detrimental effect on our lives here and in the hereafter? When we live in such states of distorted understanding, there is no incentive for us to abandon bad behavior.
Knowing about what Allah
and His prophet (s.a.w.s.) commanded us to stay away from is, therefore, equally important. Hudhayfah Ibnul-Yamaan (r.a.), one of the distinguished companions of the prophet (Sallellahu Alaihi Wa Sallam) said: “People used to ask the Messenger of Allah (s.a.w.s.) about the good, but I used to ask him about the evil, for fear of falling into it.” 
Similarly, Umar Ibnul-Khattaab (r.a.) said: “Soon the bonds of Islam will be loosened bit by bit, because people will enter into Islam but will be unaware of the Jaahiliyyah (ignorant practices that Islam opposes).” 
Let us, therefore, make it a point that we equally invest our time and effort to learn about the bad and sinful behavior because it lurks all around us. Becoming more wary about its nature and effects is needed before we can strive to root it out of our lives.
 Related by al-Bukhaaree (no. 3606) and Muslim (no. 1847)
 Related by Ibn Taymiyyah in his Majmoo’ul-Fataawaa (10/301)