There is no reason why both Piketty's and ISIS' rationale can't both be true. ISIS's eschatological religious propaganda wouldn't have nearly the resonance and recruiting power it does if it weren't for the current state of political and economic inequality in the middle east.

Also, while the Planned Parenthood shootings are tragic, they are not widespread and frequent enough to be attributed to an aggregate measure such as income inequality.

Edit: removed my comment about how we'd have to have Christian armies rising in the US to have a situation comparable to ISIS. Apparently that rubbed some the wrong way.