Bit of a scatterbrained answer, sorry for that, but its hard to explain what is going on without a lot of secondary information that people often lack

The cause of the Armenian genocide is still HIGHLY disputed, see here, although an academic consensus is starting to form towards one specific answer: The Balkan wars and other massacres/genocides by christian powers directed towards muslims. The theory goes something like this: Somewhere around one forth to a third of the inhabitants of Anatolia at that point were refugees, or decedents of refugees, be it from the circassian genocide, or refugees from the balkan wars (a good primary source for their treatment would be the carnegie report on the balkan wars). While a large amount of the perpetrators of the massacres were not refugees themselves, there is some preliminary evidence that shows the perpetrators, but not necessarily the foot soldiers, were if not refugees themselves, they were deeply affected by the balkan wars and previous refugees. In fact it is a common sentiment and part of the reason why the genocide is still denied in Turkey that because everything from the Cretan turkish population shifts to the Macedonian muslim expulsions are completely ignored by western academics, and to an extent this is true. But back on topic, the feeling in the country was at the time that the country was under siege, and at risk of colonization so to speak, and one only has to look at the treaty of sevres to show that their perceptions were not unfounded, as the intention was less based on ethnic self determination as "liberating" christians at all costs, even in areas where they lack the majority.

This contributed to a sense that Anatolia was a land for muslims, because lands that they rightly or wrongly perceived as theirs, were being annexed into christian states and ethnically cleansed of muslim populations, with forced conversions, massacres, colonization, and explusions galore. It was a systematic policy by many of these states. Now, the next part of the puzzle is how the fuck does the balkan wars, which was done by bulgarian, serbs, and greeks factor in? Well, besides the greek genocide, one must understand that the ottoman empire, while becoming increasingly turkish centric, was still fundamentally a muslim state with islam as a defining characteristic, rather than ethnicity. There was an attempt to shift towards an ottoman identity, but this largely failed. So if you perceive yourself as muslims, then the counterpart is that the christians are but a singular force, at least when it came to this topic. And there was an Armenian brigade that fought in the balkans wars on the bulgarian side, so from their view, Armenians essentially were part of the same group as the perpetrators of the balkan wars.

So, now we have established that the armenian genocide was influenced by the balkan wars and that the armenians and other christians were seditious backstabbing bastards who were going to lead to the colonization of muslim lands, at least according to perception. Then you add in the first world war. This was already a time of life or death for the empire. Now the propoganda of the first world war was still attempting to create a national identity for the empire, something that while I mentioned muslim was an important element, was not totally solidified. Look at Ottoman Propaganda and Turkish Identity: Literature in Turkey During World War I for more info. But the fact of the matter is that everybody knew of the horrors of the massacres against muslims, or had heard stories of it, so it was not super hard to turn around and say: Kill those guys, they are trying to destroy our state. The propaganda was largely word of mouth, as the book states, the real impact of propaganda was minimal as the literacy was so low. So in summary, the horrors of previous massacres against muslims, and the perceived threat of a colonization of anatolia, caused them to view a group they already saw as a threat, aka the armenians, as exceptional and allied with the other powers. It was only a step up from the earlier massacres too, only slightly more horrific than the earlier examples, just systematized, so the genocide took place.

Edit: It is probably best to view this as an ethnic conflict of escalating tensions, more akin to the bosnian genocide, where they kind of blamed the bosnians and started killing people, although they were not directly responsible for the mess that they were in, they were lumped in with the croats, and in a lot of senses this ethnic tension was the fault of the mother state, aka Serbia. There was inter ethnic fighting focused between groups outside the target, and that caused them to be associated with the other side and killed. And at certain points in history, such as during the second world war with ustasha (collapse of the Ottoman Empire) , yes, the Bosnians (armenians) did do some shit, but that they in no way deserved what happened. The comparisons to the holocaust are often pretty bad, and its best to compare it to an inter ethnic conflict genocide. Especial since it was one of perceived threat rather than of purifying the state, the jews were not touched for example (and during the balkan wars were often killed for being perceived as too close to the ottoman Empire), nor Arab christians, while the holocaust was against all minorities. The Ottoman christian genocides are a lot more complex than they are often portrayed. Especially since they were localized. One quote by an armenian minister that I cant find for the life of me at the moment is essentially "Comparisons to the holocaust are bad, imagine if it was a holocaust, but just against jews in Germany, not austria, just germany proper, and they were told to walk to Poland with no food, with the intention for them to die, but if they got there they were home free more or less" Its a really complex subject with a lot of biased scholarship on all sides.