I think much of my reactions regarding Jon Sims is colored by the tenth episode of the third season of the Trollhunters series of the Tales of Arcadia saga.

ASK AND YE SHALL RECEIVE
To preface this, I need to address that I am aware that the genre and audience of both series are vastly different. Likewise, the way they go about showing the themes I’m mentioning will be vastly different. However, I don’t care.
First off. Lets talk about power, knowledge, and consent.
Jon and Jim were given incredible powers and responsibilities; Jon as the Archivist and Jim as the Trollhunter. Jon and Jim were given both limited and unlimited information regarding those powers and responsibilities; Jon was being groomed and lead around while Jim was being trained and mentored. Neither Jon nor Jim knew what would happen when they took on those powers and responsibilities; Jon wasn’t aware that he would become the kind of thing that traumatized him as a child and Jim wasn’t aware that by picking up Merlin’s amulet that he was signing up for a hidden war. The main difference between the two’s powers and responsibilities is simply that while Jon’s were sinister in nature, Jim’s weren’t. Jon, despite his clear desire to Do Good, did damage. Jim, on the other hand, took on a mantle of Good and succeeded in that effort.
Before I progress to the discussion of how Merlin’s Evil Bath Bomb is parallel to Jonah’s Evil Chain Email, let’s talk about them.
Jonah Magnus is the titular Magnus who’s archive Jon works as Head Archivist. Merlin is who Jim fights for the glory of, calling his name to don his armor. For a lengthy duration of both series, while the implications of the two are brought up at times, we’re not under the impression that we’ll ever interact with or learn much more than we do of either or their reasons for what they did. At least until the final seasons of both series, thereby coloring everything prior. Jonah made his Institute, we learn, to effectively become immortal. He dabbled in eldritch things he shouldn’t have and became corrupted from his lust of immortality and power. Merlin, as he is the Arthurian figure, was a wizard who ostensibly Did Good. His goals were to protect both Trolls and Humans from powerful threats.
To facilitate their goals, Jonah and Merlin used Jon and Jim as pawns. Chess pieces in a game neither knew the rules to until it was too late. Whatever evil Jonah sowed and whatever good Merlin did was through them and their numerous predecessors.
Throughout The Magnus Archives, Jonah (through his possessing of Elias Bouchard’s body) tells Jon that he has choices. That Jon is the only one who is responsible for the choices he makes with or without the information given to him. Starting with him accepting the position of Head Archivist of The Magnus Institute. A title he didn’t know would lead him down a path of doing inhumane things. A title he didn’t know would mean he was, in effect, no longer human at all.
Once Merlin come on the scene towards the middle of season three of Trollhunters, it’s clear that he has ulterior motives and plans for Jim. That Jim, as the first human Trollhunter, was never supposed to stay human. During the aforementioned episode, Merlin espouses on Jim’s choices. Saying that this final decision was his and his alone, that he couldn’t force him to even if he tried. In the end, Jim does go along with the plan of shirking his humanity to end the war for good.
This? Is bullshit.
Neither of them would or could have ANY idea what would happen after those pivotal moments. In what world is becoming an eldritch being that eats peoples fear an expected outcome to taking a promotion? And why would him reading a world-ending statement, like he was forced to do under the likelihood of death, be in any way his fault? In what world is leaving the choice to become more capable of preventing an Eternal Night to a SIXTEEN YEAR OLD, in any way okay? In that same vein, why would the power to stop a war be given to a SIXTEEN YEAR OLD anyway?
So what I’m saying, what summarizes this all, is that Jon and Jim were groomed into what they became and I can’t fault Jon for anything he did as a result of that.