Can I withdraw an accepted paper if the fees are higher than expected because the proof is longer? I have received the acceptance of my paper from Editor board. However, the print charges is much higher than I expected. My original manuscript contained only 12 pages but the proof (final version), which was sent to me contains 24 pages. So the print charges are doubled.

Now I want to withdraw this paper. Is it ethical/acceptable/legal?
You can withdraw the paper. 

That said, it is expected that you are informed about the rules before you submit. Unless those prices/editing practices were hidden upon submission, it would not be professional to have your paper reviewed by 2-3 experts, evaluated by an editor, perhaps multiple times, and then withdrawn because you do not agree with the publication policies. 

Your particular case makes me think that your submission was within the declared costs, and that the editing process made it longer than required.

What I can recommend to you is to explain your situation to the editor and to the publishing staff. You might obtain a fee waiver, or a discount.