Killing Scholar Visari...
I was just re-watching the Killzone 2 intro that I found on my computer and I started thinking what a mistake it was to kill Visari, he was such an awesome evil character and voiced so well.
Making the Helghast just like Nazis, I can go with that
Making Rico extremely annoying, I can somewhat handle
But killing Visari! aka Brian Cox... worst Killzone story mistake in my opinion. Brian Cox performed both Killzone 1 and 2's intros into the higher realms of awesome gaming voiceover sauce. They were just pure epic, he seriously carried it just by the voice acting :D
Just see for yourself
Killzone 1 - http://www.youtube.com/watch?v=u5EVH3fWKQQ
Killzone 2 - http://www.youtube.com/watch?feature=iv&v=ir5EavFCHvw&annotation_id=annotation_841185
Killzone 3 - http://www.youtube.com/watch?v=hudc7npDjdA&feature=related