I would think that who is destroying it could be a factor.

But in any case, it is not a sure fire way to revoke a will.  Less than 60 seconds with my favorite search engine confirmed that destroying a true copy of a will does not always work.  There was specifically a case in Canada illustrating this.

If this is intended as a plot device for a story, go for it.

If you (or someone you know) are tempted to toss someone else's will in a shredder... just don't.

If you want to change your own will back to a previous version, consult a legal professional about the correct way to do so.