Who’s going to give baya to whom?

ISIS favors cleansing the Sunni population through violence, as well as fighting non-Sunnis, while al-Qaeda prefers to treat local populations less harshly, slowly ramping up their strict interpretation of sharia after consolidating gains.

In depth explanation of the split between the seemingly consanguine groups, from the Washington Institute for Near East Policy:

 The War Between ISIS and al-Qaeda for Supremacy of the Global Jihadist Movement