It is common and accepted "practice" for Sith to want to kill each other and take over power. It is a result of their inward, selfish philosophy. Sidious knows that whatever apprentice he has, the apprentice always wants to kill his mater and assume power. Basically, it is the theory of natural selection; "survival of the fittest". If Sidious's apprentice is more powerful than him, then he will accept his fate... Of course, this never happened (until Ep VI), as Sidious was superior to any Sith apprentice/Jedi he encountered.