Joe Kennedy was anti-English, that's clear. But to pretend that Kennedy is responsible for the Second World War (by somehow inhibiting US efforts to back the UK), seems more than slightly hyperbolic.
My premise as that if you had to choose one (American) individual, he was as good or better than any I can think of. Very often in history, one person has been the critical mass inciting or inhibiting a key event. FDR went as far as violating the law to assist England prior to 12/07/1941. Joe Kennedy worked hard to influence public opinion in order to soften the image of the Nazi regime and prevent any steps towards war. I don't pretend to have all the answers, but I am certainly not the first to point the finger at him as a key culprit. Any way you slice it, Joseph Kennedy was certainly one of the most vile Americans in history.