yes I have seen situations where if you kill key npc's, the game
give's you a "you must reload the game" message. Apparently there
is a grand story that must be followed.

If I go completly kill crazzy and be wanted all over the place. How will this afect the game.

You can kill whoever you want.

If you want to complete the game, you can kill almost everyone.

If you want to be nice, you only kill the people who attack you.

If you want to be Ghandi, you let yourself be killed by the first man you see, quit the game, and start warcraft3 or another game..

----------
It's all up to you.

If you kill everyone before completing the game, you will not be able to finish the game as the designers intended...

Now, that being said, there are a couple a mods out there that will allow you to kill everyone and be able to finish the game...

at the pressant time I have a lvl 39 war lord and am working on the
imperial jobs. most of the mage and fighter stuff is done. I have not yet proven that I am the chosen one to those tribesmen in the midle north coast.

If you want to kill everyone. finish your game first!