Why was every person attempting taking the life of a President in history either Marxist or Crazy?

Because in our system, the death of one person, no matter who that person might happen to be, does not fundamentally change the system. This is not true in Marxist, totalitarian regimes, that are largely built on the personality of one charismatic leader. Furthermore, it takes a person so delusional that they do not understand this to truly believe that the system will change because one person dies. It is generally then these who attempt assasinations. Other radicals, who understand the framework of our system better, understand that they have a better shot at changing that system by working within the system.