In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Rabee Bin Haadi [may Allaah preserve him] said: Nowadays they say about the Salafiyyeen, “They do not pre-occupy themselves except with reviling [others]!” Ali Ibn Abee Taalib [radiyallaahu anhu] fought against ahlul bidah- the Messenger [sallal laahu alayhi wasallam] commanded him to kill the khawaarij. [ref 1] The Messenger [sallal-laahu-alayhi-wasallam] said: Kill them….for I were to meet them, I would have killed them in the manner the people of Aad were killed [Bukhaari 4351] http://www.salaficentre.com/2011/10/shaikh-fawzaan-the-methodology-of-ahlus-sunnah-in-their-combat-against-the-khawaarij/
As for warning against [ahlul bidah], then indeed the Messenger [sallal laahu alayhi wasallam] and the Sahaabah warned against [them]. The Imaams of Islaam warned against [them] and they held that as an obligation. One cannot be a believer who is sincere to the Ummah except if he warns against [them] and gives clarification. And if not, then he is a concealer of knowledge – deceitful in the sight of Allaah and deceitful to the believers.
Do you think that the one who remains silent about this painful situation and behaves deceitfully towards you is a sincere adviser to you, and that he wishes you happiness in this life and the next? By Allaah O brothers! He is not. By Allaah! We do not conceal knowledge. I give advice and by Allaah we have written warnings against them [i.e. ahllul bidah]. We seek Allaah’s refuge; we ask Allaah to grant us well-being [by protecting us from] forbidden desires.
[Source: Marhaban Yaa Taalibal Ilm’ pages 27-28. Abridged and slightly paraphrased]