As a private citizen, you can’t go around negotiating on behalf of US

John Kerry denies claims he fed Israeli information to Iran.  This is very serious stuff, if true.  That John Kerry either as Sec of State, or as a private citizen who was still receiving classified briefings as a former SecState, passed on military intelligence to the Iranian FM.   Either Kerry was actively working against Israel while Secretary of State, or he was actively working against the US Trump Administration while he was a private citizen.