As far as I know killing oracles is completely optional. Maybe a trigger for another chest is not killing a single oracle. Before the Templar you can simply run to the cleansing area. During the Templar fight you have the relic to cleanse. During the Atheon fight the oracles are to get times vengeance, which again is optional. Once Atheon teleports 3guardians, they can run out with the relic and you won't succumb to the oracles. The only time you have to kill one is when the vex take control of a sync plate at the gatekeepers, but that oracle is red... Because it's the only exception? I'd think so. I haven't had the chance to try this idea but I think it's something worth looking into. Edit: so the oracles in Atheon room are all red and those in Templar room are all yellow, not killing them can still be a very plausible trigger for a spawning a chest tho.