What did John F. Kennedy do bad?

Answer 1

In his personal life, he was an inveterate womanizer. Aside from the moral questions, his flings with foreign operatives posed a security risk that might likely have led to impeachment if he had lived to serve a second term.

As president, some his decisions can be questioned, but I do not think any were clearly bad. He pushed the US to the brink of war with the USSR. As it turned out, the Soviets backed down , but if they had not, we might have had WW III start in his term-- did we gain as much as we risked? Relations with Cuba are still suffering from his policies there.

Answer 2

Yes- Kennedy was tragically killed while still young by an assassin's bullet.