@dend08 They don't seem to have a way of telling that the energy has been vastly depleted, without going into the vault themselves.
Also for everyone wondering how they thought they could get away with it, its quite simple, He broke the protection array inside the vault [regardless of the reason why he did it being justified] they also didn't plan on him putting up much of a fight or so many other witnesses showing up. And for those wondering how they can accuse him of stealing something he has full rights to have they more then likely planed to plant evidence [Other artifacts] on him as proof he stole letting them freely kill him.
They feel they can get away with this because they have rank i the clan and also are the ones in charge of protecting the vault, its also just a trope for this type of story that one of the clan elders would get in the MCs way and is either killed/banished from the clan/ or politically out maneuvered leading to them no longer having the rank to freely target the MC.