Santiago was justified to do what he did.

Throughout S2, we learn that Santiago’s maker was killed under Armand’s order for breaking the Great Laws. Santiago alludes that it was a minor issue that got his maker locked in a box indefinitely.

It is hypocritical and goes against the Coven Master’s ethics to turn a blind eye towards Louis and Claudia. After all, they broke four out of the five Great Laws.

Armand shows that he is not willing to punish Louis or Claudia so I can understand Santiago’s frustration.

Combine that with a poor work culture at the theatre that Armand was not willing to address and I can understand Santiago leading a mutiny.