he does not want people who knew him harmed by what is in his papers.”
If Souter was exemplary in his personal life and in how he conducted himself as a officer of the courts, who could be hurt by it?
Not that I am any fan of Souter but no man that rises to the highest levels of political life is a saint, they all have written, said or done things that someone can use to tarnish their reputations.
The people that advise the Justices write to in the expectation that their correspondence will be kept in confidence. Once the Justice’s papers are opened to historians all of these letters will be publicly available. Justice Souter is try to respect the privacy of those who advised him in expectation of his discretion.
His sealing his papers for 50 years is not surprising or unusual. Most presidents have done it for at least some of their papers.