Also worth taking note of here is the fact that Erik is pretty much heavy breathing in every single scene.

Then, we've got the torture scene where Henry decides to throw out some choice statements about Erik that very clearly do their job in getting to Istvan:

We see more of the heavy breathing from Erik coming up again the last time Isterik see each other. Erik is clearly someone who is at all times trying to hold back emotionally, and it seems that Istvan (or things related to Istvan) directly correspond with a loss of that control.

As you already know, we get a lot of Isterik moments in the scene where Henry kills Istvan, but in case you wanted to see the highlights, I've got you covered:

What really struck me here is how Istvan acts as a fairly direct mirror to Erik in terms of emotional control. Erik seems to be the one thing that can make him fall apart in two seconds. He's so calm in most of that scene, but if Henry starts talking about how he'll kill Erik, Istvan starts screaming.

We fast-forward a bit. The scene where Erik is given the news about Istvan is like a masterclass in physicality from him. We upgrade from breathing to snorting and snarling to everything else. Huge props to Jim High here on turning Erik into what is effectively an animal in that scene.

At the Italian Court, Erik has to actually hold it together because he's literally commanding a bunch of men. And even there he struggles substantially in the face of all of Henry's jeering:

Yet again, we fast-forward, this time to the scene where Henry can fight Erik. But this time, it's to my favorite part of the Isterik devnotes by far. Namely, this: