What you describe is known among historians as the "brutalization" thesis. In what follows, I'll give an overview of the idea and of some critical reactions to it. I'll also offer some reflections on its relationship to ethno-nationalism.

The seminal text for the "brutalization" thesis is by George Mosse (himself a WWII veteran) called Fallen Soldiers (1990), which held that the experience of wartime violence led WWI veterans to embrace violent political ideas and thus to the political radicalization (or brutalization) of the interwar period, in particular to the rise of Fascism and Nazism, in turn leading to the violence of WWII. That isn't to say that this was the first time the notion was expressed - immediately after WWI politicians from Britain to Russia were worrying that returning soldiers might engage in violent crime or join violent armed groups. And the idea has been applied to many conflicts - see Kathleen Belew's recent work on the Vietnam war and the rise of the far-right.

Historians have questioned, however, whether the war experience itself is sufficient explanation, given geographical discontinuities across Europe: politics was "brutalized" in some states but not others (see the work of Antoine Prost on France and John Laurence on Britain, for example). Certainly, in Eastern Europe, WWI veterans were key in the civil wars and revolutions in WWI's aftermath. Scholars have looked to explain this geographical divergence by recourse to French and British colonies as possible alternative sites of escalating post-war violence (alleviating the brutalizing pressure on the metropoles), or to the power and culture of defeat (in reality or perception) in states like Germany or Italy.

Even in Germany, Benjamin Ziemann and others have convincingly argued that most WWI veterans returned to their "normal", peaceful, pre-war existence. Richard Bessell, writing soon after Mosse's seminal text, found that the German paramilitaries assumed to be “brutalized” members of the front generation were more likely to belong to a younger cohort inspired by the front generation’s mythologized example.

In short, it is undoubtedly the case that some WWI veterans interpreted and remembered their experiences in a way that contributed to their radical politics and violent behaviors in WWII. But a lot of history had to happen in the interim. The process of demobilization, issues of welfare, unemployment, housing for veterans, and of course broader social and political trends, all shaped how prior experiences might feed into future actions.

Historians have also looked to an underappreciated aspect of Mosse's text - the notion that it wasn't simply soldiers who were brutalized by war, but societies and cultures generally.

So to answer your question, yes there were certainly individuals like Dirlewanger whose experiences in WWI are key if we are to make sense of their beliefs and behaviors. But there is much more debate and much more scepticism among historians about positing the experience of WWI violence as a major or central cause of WWII, IF what you mean by that is the specific experiences of soldiers and the impact of violence on their psychologies. IF, on the other hand, we mean the causal links between WWII and the cataclysm of WWI generally, then that is another question. The violence of WWII is unthinkable without WWI.

And that leads me to your comment about ethno-nationalism. The ethno-nationalism of WWII was in large part due to the events of WWI. This aspect is sometimes overlooked because the history of WWI is still so often taught through a Western-centric lens. But one of the main effects of WWI was to turn a continent of empires into a continent of nation-states, and this was particularly the case in Eastern Europe, in what is sometimes referred to as the "shatterzone of empires", in the lands of the collapsing Austro-Hungarian and Russian empires. This is also the region in which the most brutal violence of WWII (and most of the Holocaust) took place.

This is quite a lively area of research at the moment. (It is something I am working on also). We have plenty of analyses of the multitude of civil conflicts and revolutionary movements in the immediate aftermath of WWI and the role of WWI veterans in those conflicts, but there is still much to be done, and much to be done on the potentially "brutalizing" shadows that those conflicts themselves cast upon the region. One important narrative, for example, is that the ethnic violence of the post-WWI years, as nationalist movements tried to carve out homogeneous nation-states in this region of complicated demographics, led to a pattern of reprisals and resentments that was ignited again by WWII. Here too, we are left to examine the particular circumstances of the post-WWI period in order to understand how the violence of WWI fed into WWII.

Another area where we might examine these causal threads is with the "stab in the back" myth - the antisemitic conspiracy theory that held that Jews and socialists back in Germany had betrayed the German Army in signing the armistice in November 1918. This myth was part of the anti-Semitic thinking of Nazism and became an integral part of Nazi propaganda once they took power. It is an example of how WWI facilitated the crystallisation of already existing antisemitism into virulent ethno-nationalist movements, and central to its power was its appeal to the heroism of soldiers and the hardship they had endured for their country (an appeal which resonated not only with (some) soldiers themselves but with others inspired by them or their mythologized image, as Bessell showed).

To conclude, there are many factors at play in shaping whether and how experiences of violence contribute to future behaviors. The experience of violence itself is amenable to various interpretations, can be remembered in different ways. For some individuals, and in some contexts, the memory and/or psychological impact of WWI formed part of the motivation for the atrocities of WWII, but this itself was a complicated and diverse process - a lot of history happened in the intervening years.