that was pretty well explained darth (then lord zash) was thanatons underling and not only was she pro-freedom anti traditional (something that ticks thanaton off by it's self.) she also disobeyed a direct order (do not kill skotia) with the way sith think disobeying an order from a superior means she and every one associated with her has to die. the problem is he couldn't prove it and according to the tradition he likes so much that means she should get away scott free (it's only murder if your caught). So thanaton had a problem on the one hand he knows zash is antitradition and has killed skotia in defiance of every thing he holds sacred on the other he can;t actually prove it so everything he holds sacred says he has to let it slide.
you on the other hand had many witnesses see you go into the temple to kill zash. Which makes you an easy scapegoat to replace zash as the target of his rage at being disobeyed and you broke tradition (becuase of your supposed murder of zash being so open and common knowledge if you will.)
There is no stupid tradition, it's all nonsense.
Betrayal and backstabbing is the soul of Sith. Sadow murdered his master, Freedon Nadd betrayed Sadow, Exar Kun destroyed Nadd. Many many Sith in TOR's Empire kill each other's underlings or their superiors. It does not make sense at all. And he didn't destroy Zash right way(He totally could).