OK, so remember that quite fun Vault of Glass raid that was out since vanilla? Yeah, quite fun indeed isn't it? While a great raid with interesting mechanics, there is, I have found, a HUGE plot hole. So the Vault was basically to imprison Atheon because he was deemed dangerous by the Vex, right? So we have to go kill him because the Vex won't do it themselves, but why? If they were unable to, that's understandable. But why were the Templar and all the other Vex trying to kill us? Wouldn't it be easier if we just killed Atheon so they didn't have to guard all that Vault and stuff? They could turn it into a factory or something to create millions of vex units. What do you guys think? Silly or legit?
TL;DR: Atheon was a prisoner of the vault, so why did the Templar and other geezers try to stop us from killing him? Wouldn't it be a favor?
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The vex in the black garden send data to the vault of glass in order to develop weapons, like the oracles that can erase anyone from existence. The vault of glass is a research facility for the vex to develop those weapons. Atheon is the vex unit receiving this data, not only from our timeline but from other timelines as well. So we venture into the vault to kill it and halt the development of their weapons