there definetely is a tradition in the sith of when it's acceptable to murder your superiors and when it isn't acceptable several dark lords have ignored it true but you'd be surprised how many haven't. , I/zash/apparently you say its fine to ignore that tradition thanaton believes that ignoring that tradition will ruin the sith and turn them into indescrimanate murderers something which they really aren't at the moment.
there are rules to follow when you want to murder your superiors zash didn;t want to follow them but did so she was safe from thanaton. you as her apprentice didn't follow them so you became the suragate target for his rage at zash.
Where did the tradition come from? Sith is all about betrayal and backstabbing, Thanaton liked Naga Sadow and Tulak Hord and they both did such thing.
Darth Baras let his apprentice did the same thing many times. Darth Mortis, the enforcer of law also accepted Nox and Karrid after they defeated their rival. Thanaton's "tradition" is totally nonsense.