Original post by ZamestanehBecause it's a sectarian conflict which has been raging on since before ISIS - after the Iraq war, a number of the disillusioned Sunni soldiers who had been dispanded joined resistence groups and terror groups. These groups fought against government and occupying forces, as well as Shia militia in the south. Some of these groups would bomb Shia populated areas, shrines etc, and equally these militia would carry out retalition attacks. When ISIS formed out of one of these terror groups, they simply carried on doing what they had been doing for years i.e. attacking Shia. The Shia militia used to be smaller, but as ISIS has grown, they have received increasing amounts of funding and arms from Iran, and therefore they have grown massively, and it is arguable that they have more power than the official Iraqi army; their longstanding grudges remained the same, thus they carry out attacks and abuses in retaliation on a larger scale now, and again, it's not limited to killing ISIS and its supporters, because this is a pre-ISIS grudge.
Because of this hatred and suspicion, it is not abnormal for the Iraqi soldiers and militia troops to have contempt for the people they are 'liberating' - if they aren't ISIS soldiers, then they are ISIS supporters in their eyes, and if they aren't ISIS supporters, then well tough luck I guess.