It's possible it has to do with the amount of Bhaal's 'essence' the body possesses. At the end of TOB it is mentioned that Imoen had a lot less than CHARNAME's does, so maybe that has something to do with it. Presumably Sarevok would have had a large amount as well.
Comments
Sarevok - "I will be the last! And you will go first... Heh hehehehe..."
::Splat::
Sarevok - ...
::Victim does not dissolve::
Sarevok - "...whoops."