[Citizen Blade]

A President could walk out onto the White House lawn, pull a gun and off a Girl Scout, and there is nothing 'the public' could legally do.

AFAIK, a President is not immune to criminal charges. If a sitting US President were to commit a crime, such as fraud or murder, there would be nothing to prevent him from being charged.

[Sharrukin]

The OLC memorandum concluded that all federal civil officers except the President are subject to indictment and criminal prosecution while still in office; the President is uniquely immune from such process.

http://www.usdoj.gov/olc/sitting_president.htm