Killing for a higher cause of one's community is most common. Out of ten Muslims responsible for killing Christians, Hindus, or Ahmedis, only two to three are booked and one is convicted, if at all. The same holds for Sunni killers of Shias and vice versa. No, not in nine out of ten cases. In Pakistan, you can get away with murder, literally.
Ethnic killing is carried out for supremacy in the street.
It is a function of mono-ethnic tendencies in the state structure, in the form of Punjabism for some today or Mohajirism for others yesterday. Few among those who killed their ethnic enemies were brought to justice. It is politics, not law, that prevails in the end. It is considered against political expediency to pursue ethnically motivated killing. Bureaucracy in Pakistan contains elements within its ranks that ration out the writ of the state only selectively, in favor of sectarian brothers or co-ethnics among criminals.