The POWER of the Dark Side
In Return of the Jedi, we see Vader telling Luke how he doesn't know the power of the Dark Side and how he must obey his master. In addition to his jelousy of Obi-Wan, I think he was afraid of dying when he realizes how powerfull is Palpatine in ROTS. That scene may well be the scene when he confronts him( discussed in my previous thread)...