Why does the Dark Presence attack Alan?

Throughout the game, Alan Wake is attacked by creatures that have been possessed by the Dark Presence found under the lake, where Wake and his wife stayed in Bright Falls. By the end, though, we learn that the darkness needs Alan alive, in order to

write his novel to allow the darkness to escape.

thus why it kidnapped Alice at the start of the game. If so, why is it constantly trying to kill him?

Because when Alan wrote the story he knew in order for it to work, he had to put himself in danger with close calls, etc. The pattern established through the game is that the dark force needs someone creative to free it, but so far no one has been able to give it the story it needs to escape its prison. When Alan explains at the end of the game he mentions that he not only needed to write a story, but it had to be a good one. Without real peril, the dark force could not break out, nor could it be defeated either.

At least that is how I remember it.