It's kind of a misnomer that Suuni and Sufi are in some sort of conflict generally. It's hard to describe, and I'll probably do it wrong, but think of Suufi's as being far more internally focused on a personal relationship with God, whereas Suuni's are more focused on the communal aspects of an organized religion - but only to the extent of each community, and Shia have a world-wide hierarhcy approach. So in a *very* rough way think of Suufi as being monastic/meditative/contemplative Islam, Suuni as being a Protestant Islam, and Shia being Catholic Islam. (A *very* rough analogy but using western concepts to help in explaining.)
Groups like ISIS hate everyone who doesn't submit to their will. It's not a hatred rooted in religious tenets, they just claim that for local social currency. It's a power grab. They cloak themselves in the religion of the areas they arise in because it allows them some attempt at legitimacy (which they usually squander.) So they'll kill Suuni, Sufi, Shia, Christian, Druze - you name it. With equal measure.
This is a real split, perhaps the defining one, in the salafi-takfiri world between groups like ISIS and their allies in Boko Haram, Abu Sayeff etc. and Al-Queda and their allies such as Taliban and Haqqani. Al-Queda version of salafi-takfirism forbids the killing of *civilian* Muslims in war through terror. (They still do it, but they don't advertise it or brag about it.) ISIS does not forbid, and almost relishes in it.
