Why are this twin brothers always fight and persecute themselves?

^The conflict between Sunni and Shi'a revolve round the successor-ship to the holy prophet (saws).

1. The nucleus of an unending conflict between Sunni and Shi'a to the point of killing and black-mailing, whether religious or political in nature started at the death-bed of the holy prophet (saws).
The prophet (saws) was about to write down his will as Qur'an instructed every believer to do so at his death-bed. He (saws) asked for 'pen and writing pad' to write down a document in which so long it is followed, none among his follower will ever go astray.

Alas! The community divided. Some continuously repeated the order of the prophet while others said the prophet is delirious and that Quran (alone) is enough.

According to one of the best narrator of this incidence, Abdullah ibn Abbas, he said:

The misfortune and division of muslim started that day.

Those (group) who supported that the documents of guidance should be writing down interpreted the document to be initial verbal declaration of the prophet (saws) in many occasions which says "I leave amongst you two weighty things so long you adhere to them you will never go astray; the first being the book of God and the other, my progeny, my ahl al-bayt (household)..."

Therefore, these people cling to the household of the prophet (saws) for guidance and leadership and they were known as "Shia (supporter) of ahl al-bayt (household of Muhammad)". Example of such people were companions like Bilal, Abu Dharr, Salman al-Farsi, Miqdad, Abbas, Abdullah Ibn Abbas et al.

On the other hand, the other group who prevented the materials to be writing do not even waited for the dead body of Muhammad (saws)to be buried or carried along the family of muhammad (saws) their plan before they choose a leader to succeed Muhammad (saws) at a place called "Saqifa".

Even at the place of nomination of a successor, there were great chaos that nearly resulted into riot as every individual and group present have one or two interest to protect.

2. Prior to these event (death-bed/Saqifa), the last Hajj performed by the holy prophet (saws) witnessed the presentation of Ali ibn Abi talib as the "Mawla" of every muslim. The holy prophet (saws) publicly raised his hand and declared "Of whomsoever I am his Mawla, Ali is his Mawla".

Every muslim present then, individually and in groups, continuously reported to have congratulated Ali on that day for being chosen as the "Mawla" of all muslim.

What is the meaning of "Mawla" as used in this context by the holy prophet (saws)? This simple word further divided the once united community of Muhammad. Those who interpreted the word as Leader, guardian after Muhammad (saws) cling to Ali and never recognized the nomination and leadership of Abu Bakar as legitimate successor to the prophet. These group from the beginning till date are known as "Shi'a". While those who interpreted the word "mawla" to be a mere "friend" and therefore accepted the legitimacy of Abu Bakar's successorship form the nucleus of "Sunni".

NB: The word "Sunni" from the original designation of "Ahlu sunnah wal Jama'a (people of Sunnah and community)" was never used until the time of Mu'awiyyah Ibn Abi sufyan, almost 40years after the demise of the prophet (saws).

On the other hand, the word "Shi'a (supporter)" has always been in use by the prophet (saws) himself, and especially by the muslim community during the first civil war - battle of Jamal. Some group were known as "Shi'a Uthman (supporter of Uthman)" who were on a "mission" to avenge the blood of Uthman, the 3rd caliph. This group was led by the wife of the prophet, Aisha. The other group was called "Shi'at Ali" (supporter of Ali). After the first civil war, another civil war surfaced instigated by Mu'awiyyah while his supporters were known as "Shi'at Mu'awiyyah" and that of their opponents remain "Shi'at Ali". Mu'awiyyah later changed this name to "Ahlu Sunnah wal Jama'a.