When you teach the Deen, you have to warn against that which opposes it:
1) When you teach Tawheed, you have to warn against shirk and its different types.
2) When you teach the Sunnah, you have to warn against Bid`ah and its different ways.
3) When you teach the Haqq (Truth), you have to warn against the Baatil (falsehood) and its different forms.
If you don’t, then you might just end up wasting your time. How can people know the truth from the falsehood if you don’t make it clear to them? When we say the Shahaadah, لا إله إلا الله, we are first taught to negate the worship of every other “god” besides Allaah, and then we are to affirm the worship only for Allaah alone.
Narrated Hudhaifah bin Al-Yamaan رضي الله عنه: “The people used to ask Allaah’s Messenger (ﷺ) about good, but I used to ask him about evil FOR FEAR THAT IT MIGHT OVERTAKE ME.” [Saheehain] This is Hudhaifah bin Al-Yamaan رضي الله عنه with whom the Prophet صلى الله عليه وسلم trusted his secrets but he feared for himself that he might fall into the Fitnah.
If warning against evil was not necessary, the Prophet صلى الله عليه وسلم would not have felt the need to answer his questions. If the Sahaabah felt the need to have their doubts clarified, then what about us? And Allaah is the One who grants success.