No one says Shia extremists don't commit crimes on Sunnis, but their targets are mainly Sunnis and mostly a response. Unlike Sunni extremists whom target everyone on earth, they even target other Sunnis. The Shi'ite extremists are angered by all the killings on them done by Sunni extremists, so they retaliate, sometimes a terrorist supporting village will be the target of that. For example look at the 1700 people massacred near Speicher in 1 day, you think they have no family members whom will retaliate ? even though they barely did retaliate.
This kind of retaliation is done by everyone on earth when the situation becomes bad, take Algeria in the 90's as an example, all Sunnis there.
List of massacres during the Algerian Civil War - Wikipedia, the free encyclopedia
Who knows how many massacres the army had to carry out on terror supporting neighborhoods to rid the country of them. Sunni Islam is infected with ideologies supporting terror. TTP massacred 100+ kids, many of them will return to playing poor villager/farmer as that's how they operate in guerilla warfare, should they massacre them or leave them ? maybe massacre is better.
And they are massacring TTP, there is no other way to defeat terrorists using guerilla warfare.
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