All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is prohibited for Muslims to fight each other or kill one another, and whoever does this commits a major sin. Allah Says (what means): {But whoever kills a believer intentionally – his recompense is Hell, wherein he will abide eternally, and Allah has become angry with him and has cursed him and has prepared for him a great punishment.} [Quran 4:93]
The Prophet said: “A faithful believer remains at liberty regarding his religion unless he kills somebody unlawfully.” [Al-Bukhari]
Besides, Abu Bakrah, narrated that he heard the Prophet saying: “'When two Muslims fight (meet) each other with their swords, both the murderer as well as the murdered will go to Hell-fire.” [Al-Bukhari and Muslim]
So, Muslims should stick to the Sunnah and not let Satan divide them into conflicting groups killing each other.
In fact, Satan's ultimate objective is to preoccupy the Muslims by internal problems, hindering them from fighting against their real enemy.
Dawah should be revived. So, all the Muslims who stick to the Sunnah should call their brothers who neglect some of its basics to return to the true teachings of Islam. If they accept this call, that will be good for them and for Islam, and if they refuse it, their fate is decided by the Muslim ruler. As for their fighting each other, it does not help achieve the objective and the only party that benefits from it is the enemies of Islam.
For more benefit For more benefit on how Ahlus-Sunnah should deal with disbelieving innovators, please refer to Fatawa 6283, 56164 and 14264.
Allah knows best.