Kill Sarevok and continue to do advuntures? No Problem!

There is a way to continue to do adventures after defeating Sarevok and still not complete the game.

When you enter tha Bhaal temple in the Undercity you let Sarevok attack you and just go outside the temple and kill him there.
The game will not finish and do the Final Save until you re-enter the temple.

So defeat your malignant half-brother and do some more awesome quests before Irenicus captures you! :-)