On forum: 06/06/2009
You WILL find Strelok at some point in the game, but if you then kill him your game ends.
That particular mission will never be completed if you finish the game (you don't 'beat' S.T.A.L.K.E.R. games - you survive them).
If you think you've 'done everything in this game' you've missed a huge amount. You find out who Strelok is about a quarter of the way through.
Have you been given the task to meet Doctor in Strelok's hide-out in Agroprom Underground? That's when you find out who Strelok is.
A preoccupation with the next world is a clear indication of an inability to cope credibly with this one.