Both the cast and many fans online have been referring to Essek Thelyss as a “war criminal” and guilty of betraying the Mighty Nein. But while Essek certainly is guilty of many evil deeds, he is not guilty of either of those things.
First off, Essek is not, technically, a war criminal. Actual war crimes include “intentionally killing civilians or prisoners, torturing, destroying civilian property, taking hostages,” etc. [Source] Essek has done none of those things. What he has done is, a) betray his country, his people, his den, his family, b) give away an ancient and deeply religious artifact in search of knowledge, and c) recklessly endanger the lives of people on both sides of the border.
Essek didn’t go out of his way to intentionally harm others, that was not the goal he had in mind, but he did act selfishly knowing that it was a possibility this could lead to an all-out war and not caring. He accepted that as a possibility and acted anyway. It was an evil, selfish deed, but it was not a deliberate attempt to harm. The harm was merely a side-effect.
Also Essek, like anyone, is a product of his environment. He grew up a prodigy of arcane powers, with high expectations placed on him from a very young age. One can only imagine the pressure he was on in earlier years to advance arcane study in his field, and then finding that study blocked by the same people who held those high expectations, all due to their religious fanaticism. What may have started off as a mere roadblock would eventually become a disdain or even a hatred of the religion of his people — of the only thing standing between him and what he has been told for so long is his purpose in life.
Second, Essek did not betray the Mighty Nein. All of of this was done years before the Mighty Nein had ever even known about the beacons or the conflict between the two nations. The only thing Essek is guilty of in regards to the Mighty Nein is of keeping it a secret from them this long.
In fact, the only reason Essek was even meeting with the Martinet that day in Nicodranas was most likely to ensure that the correct beacon was passed over to the Dynasty. It was an attempt to fix his past mistakes.
It could even be interpreted as Essek helping the Mighty Nein secure the peace they sought by fixing an oversight of theirs. In the conversation they had in Rosohna, Essek stressed how important it was for the Bright Queen to never discover there was a third beacon. They all knew how obvious it would be to the Dynasty that the beacon they had seen at the Vergessen Sanatorium was a different one, and yet they did nothing to correct it. (Doing so would have revealed to the Assembly how much they knew and possibly painted an even bigger target on their back so perhaps it was for the best, but in return Essek had to be the one to stick his neck out.)
Regardless of his past or his reasons, Essek is still guilty of treason and of sparking the war between the Dynasty and the Empire (and thus causing the deaths of perhaps thousands of people). He is guilty of empowering the Cerberus Assembly with new magic they had longed to get their hands on — and DM only knows what they hope to achieve with that. But it’s important to avoid accusing him of things he is not guilty of. Especially when Veth and Beauregard keep talking about punishing him, as if they have any right to do so.