Why is there violence between Muslims when all of them bear witness to Oneness?

The Sharia emphasizes the sanctity of a Muslim's blood, property, and honor, and strictly prohibits harming them in any way. Penalties have been prescribed for this purpose. Disputes between Muslims should be resolved by calling communities to adhere to religion, spread love, offer mutual advice, encourage patience, forgiveness, and detachment from what others possess, and promote peace and mutual gift-giving. Avoiding sins also contributes to eliminating hatred. In case of conflict, there must be bodies for reconciliation between disputants and deterring the oppressor, as reconciling differences is among the greatest deeds. It is permissible to repel an aggressor in defense of oneself, family, and property, but with the least forceful means possible. It is not permissible to kill them if they can be repelled by other means.