If you want to be really subtle, and are down with Psionics, Psion Telepath 20 is a really effective killer. The great thing is that many of their powers look like the person died of natural causes, plus they have all kinds of nasty mind control, and access to plenty of utility powers. Telepath//Psychic Warrior/Slayer could be very good, or just Telepath//Something with Int Synergy (Factotum, Warblade, Duskblade, Wizard, Beguiler...the list goes on)