i always pictured The Watchers murder of Mazzarin as being much more violent.
I believe it was seraph who said that Mazzarin would not likely be caught on his own. But, someone with that much power doesnt exactly need a guard to escort him around.
I like the idea of this happeneing right after he collapsed the shrine on Sinis. I like to interpret this in oen of two ways:
1. He set out on his own, to defeat the evil Sinis. The Watcher took advantage of this, and caught him unawares.
2. He set out with a large army to destroy sinis, but the troops were destroyed in the battle (he probably had the dispersal dream-it seems pretty common among sorcerers). Mazzarin then got ambushed by the watcher when making his lonesome way home.
Remember, the watcher is an ancient evil. Hes no stranger to cruelty-look at the thrall, twisted husks of living things, risen up to kill their own friends and neighbors. I believe that Mazzarin was spent, and that the Watcher was fresh for battle, and he just destroyed mazzarin, utterly and completely. Then, he probably reincarnated both mazzarin and sinis into shades.
I certainly dont think of the watcher as being, in any way, a laid back kind of bad guy. Think back to Flight From Covenent and the surrounding levels (TFL days)...your troops, hiding in an underground tunnel, afraid to sneeze for fear of your lives, with the watcher and his troops fanatically annihalating everything above you in the search for the codex.
God...hes sexy...