So there were a lot of things going on in that period, let's say 1915-1937. These areas were barely within the central state for a great deal of late 19th century policy (and really, some areas, up through the Dersim Massacre in the late 30's). Much of the Ottoman policy of the region was about bringing the areas more directly under the control of Istanbul/Ankara, in a variety ways including not just displacing or coopting local rulers but in making sure there were loyal (Muslim, Turkic) populations with power. This fit into pre-existing tensions between, say, different tribes and clans (the rural areas around Van were very tribal) and also between lowland and upland people.

Many groups of people obviously resisted the state's attempts at centralization and creating a "loyal" population, and the state was particularly brutal in some cases. There's of course what's known as the Armenian-Assyrian-Greek genocide (which had fighting on both sides, especially in Van--see the Van resistance), which continued after WWI as the allies promised the Armenians a state that would include Van). The dream of this independent Armenian State led directly the Armenian Turkish War of 1920, though the fighting was far away from Van. I think by 1920, though it had long been a center of Armenian life (the area around Lake Van is far more important to Armenian history than the area around Yerevan that makes up the modern state of Armenia).

After the Wars, and after the new Turkish Republic was established in the Anatolian part of the Ottoman Empire, there was a tremendous attempt by Ankara to more secularly incorporate the largely Kurdish Southeast. There were about a dozen rebellions between 1920 and 1937 by Kurdish groups. Turkish Kurds have several cross cutting cleavages (language: Kurmanji vs Zaza; religion: Alevi vs Sunni; in addition to tribal and clan ties) and none of the rebellions could unite Kurds across both language and religion. A lot of these rebellions were either predicated or followed by the exiling Kurdish notables to the West of Turkey. The best book on this (and it also covers the massacres and deportations of the Ottoman Christians) Uğur Ümit Üngör's The Making of Modern Turkey; Nation and State in Eastern Anatolia, 1913-50 (Oxford University Press, 2011). It's very readable and, though it focuses mostly on the bad things, I think it will give you a picture of your grandfather's world. Üngör sees the state's attempts to control, kill, and deport Ottoman Christians and Republican Kurds as all of a piece, calling them both "social engineering" (he did his graduate work in the Netherlands, where that's a trendy term in history/genocide studies). This is the best English language book I can think of that covers the Eastern Anatolia around 1920.

The fact that he was Yörük is interesting. If he were Yörük, I think he would be less likely to be affected by the social engineering that Üngör writes about--after all, the Yörük are Turkic and the state wanted generally to keep the Turks in place by the 1920's (but perhaps not as early as 1920--this period is just when nationalism is starting to really matter in the region). I can't remember if Üngör discusses them, but he may. Yörük, however, were nomads and the state spent much of the 19th century trying to get nomads like them under control. So their position in this period was shifting, from suspicious nomads to potential good Turkish citizens (though being Alevi still made one slightly suspicion to the Turkish state). Maybe see Selim Deringil's The well-protected domains : ideology and the legitimation of power in the Ottoman Empire, 1876–1909 (in part about how Abdulhamit II pushed orthodox Islam, which obviously would affect Alevis), though you might only be interested in a couple of chapters, Deringil's article "'They Live in a State of Nomadism and Savagery': The Late Ottoman Empire and the Post-Colonial Debate" (might be too technical; here's a full text PDF link) and Reşat Kasaba's A Moveable Empire: Ottoman Nomads, Migrants, and Refugees. This last one I think is perhaps the best book to get a sense of nomads in the late Ottoman (not much necessarily changed from the late Ottoman Period to the 1930's).

But it sounds like you think what's going on here is more personal than geopolitical. It's important to realize that around 1920 was a time of famine in the East. I believe the famine had peaked in the years before, but I'm not positive. Here's a very famous map that was published in the New York Times in 1918. Politically, the makers of this map chose to separate out Wilsonian Armenia from the rest of Eastern Anatolia, but the Armenians were starving in many regions to the South of the black oval marked "Armenia". I have never studied the issue of this famine in depth--anything involving the Armenian Genocide needs particularly careful study for I think obvious reasons--but I would be surprised if Arabs, Kurds, and Turks were not facing a particularly accute food shortage in those regions.

Blood feuds were common enough in Turkey through the 1950's, and common in the Southeast beyond that. They could have risen out of the hungry years, or something else. The sociologist Roger V. Gould has this excellent book called Collision of Wills, which argues that when there's uncertainty over social position, violence in more likely. One of the things he studied were Corsican blood feuds and he noticed that in times of political struggle, the number jumped hugely (he also noticed that while they in theory involved a large extended family, in reality they involved sons, brothers, uncles, and only rarely even first cousins). If you want a general sociological analysis of blood feuds, there are a few great chapters in that book (again, his stuff on blood feuds is primarily about Corsica, but I think the general pattern holds). In a time of utter disseray and confusion, like that area was in 1920, when no one knew the future and where they'd stand (or even which state they'd be standing in) in five-ten years, we'd expect a sharp uptick in personal violence which would mean a sharp uptick in blood feuds.

The best article I know about blood feuds in Turkey is "A Death and a Youth Club: Feuding in a Turkish Village" by Paul Stirling. It's from 1960, based on field work in the 1950's. You see again though this tension and ambiguity over rank (who's higher in the social hierarchy in this fast changing society--one of them had just returned from the military which would earn him respect in the new bureaucratic order but not the traditional village order, which was based on land ownership) lead to violence. The shooter flees down, and moves to a city if I remember correctly. Anecdotally, this is a common outcome for feuds. I've met several Kurdish people (feuding remained more common in the Southeast after it declined in the rest of Turkey) who explained their family's migration to Turkish-majority cities as originating out of a blood feud. Here's the JSTOR link to that article. Even if you're not an academic, you can sign up for public JSTOR membership that gives you access to a few articles a month. That's all I got, I'm not sure if that's what you're looking for. Maybe /u/jdryan08 can add something else (his focus is a little bit earlier than mine, but I think 1920 is still before the period he really specializes in).