Edmund spitballed the idea of hexing to Alan.
Alan, out of selfishness to keep MC alive and stop his suicidal rush to the end, agreed to hex him, justifying it to himself as a "temporary" measure until the find the demon king. However he was probably too guilt ridden to actually tell him to his face, so just did a bullshit kick him from the party thing.
Edmond then, without discussing it with Alan, took it upon himself to place a much stronger hex than necessary on MC in order to cripple his magic power far greater than necessary, doing so torn between his desire to protect MC, to protect Alan and the party's ability to grow and more nefariously to gain the title of greatest sorcerer and step into the limelight.
And I guess the plot slowly got tangled up, with both Alan and Edmond not really wanting to admit their plot, because what they did for mostly "good" reasons was genuinely a bad action and hurt their friend far more than ever helped him, leading them to take their mistakes to their grave for better or for worse.