It’s all about repentance, you see. It’s just a selfless concern for their souls. “And as for those who disbelieved, I will punish them with a severe punishment in this world and the Hereafter, and they will have no helpers.” (Qur’an 3:56) The Muslims are the instruments of Allah’s punishment in this world.
“ISIS Justifies Its War on Yazidis: We Called on Them to Convert to Islam First,” MEMRI, August 20, 2014:
In a video posted online on August 20, 2014, an ISIS commander explains that they offer the Yazidis a chance to convert to Islam before killing them. “The Islamic State does its utmost for the repentance of any infidel – Yazidi, Crusader [i.e., Christian], or Jewish,” the commander claims. The video shows a scene of mass conversion, with a hall full of Yazidi men reciting the two shahadas.