There are two reasons the game should stay 3D:
The first is an environmental feel that you're ALWAYS surrounded by Slender. Being 2D means it's obvious that he's either there or not.
The second is that there is an immense map so if you see him (preferable from a distance) there's almost always somewhere to go. You don't get trapped between, say, the wall and Slenderman.
Sorry, but this game just doesn't work. You get constantly trapped and it just isn't a "lost in the forest running away from something" game. In the 30 seconds this game can be played before committing suicide because you have nowhere else to go, I could be playing a different game, and not dying.
Another thing I noticed is that the pages seem to be written by Slenderman? In Slender the pages were written by other people warning of him. Just something I wanted to point out. And your English is fine, especially considering this is the internet.