Article I, Section 9, Paragraph 3 reads: No bill of attainder or ex post facto law shall be passed.” You cannot make an act illegal and use that illegality to convict someone if the act were legal at the time it occurred. That does not mean the defendants met the test for a valid citizen’s arrest under that state’s law. The trial will answer that. It also does not mean that what they did was not a “legal” lynching. It was.