I think there's a really simple explanation as to why Soon took the Oath the most seriously:
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Paladin


Also, I think it's telling that despite Serini, Shojo, the Draketooths and Dorukan being alerted to the fall of Lirian's Gate, Xykon could lay siege to Dorukan's Dungeon for months without Serini or the Draketooths meaning any known attempt to help. Dorukan did not ask for help and no one checked regularly enough to notice he might need any.