We used to think in this country that no one is above the law, that includes the President of the United States.  If the president can break the law, he or she becomes a dictator without the checks or balances of a Congress or a judicial branch of government.  There is nothing in the Constitution or legal history for granting the former president immunity.  
In fact, if Trump specifically was engaged in insurrection or rebellion on Jan. 6, 2021, using lies and misinformation to encourage the invasion of the United States Capitol, he can be held responsible.  He would be the first president to be charged with an attempt to overturn a presidential election. The lower courts ruled that the president has no immunity and there are no precedents for immunity.
The concept of immunity is entirely the creation of this Supreme Court.  The plurality of this court is made up of justices selected by the indicted defendant.        
Gerry Snyder
Ely