If this individual did go to the police, it is unlikely that they would get involved, unless there was a specific threat in the wording of the letter, which is classed as 'threatening behaviour' - an arrestable offence.
I do hope that you have considered your proposed actions over this issue.
i think most posters agree that nothing good will come out of your proposed actions - for anyone, including you.
The best action is to park this and move on - as hard as that may seem.
i think we all understand your desire to hit back and repay in kind some of the hurt you have suffered, but your wish will not be fulfilled, and it will only delay the healing process you need to get past all this.