Extremist does not mean much, just that you are extreme in your views. You can be an extremist at everything. Marx was an extremist communist, so was Lenin. Putin and Hitler are right-wing extremists. They are also very nationalists, as is often the case with right-wing extremists. On the other hand, the left-wing extremists like Marx or Lenin were not nationalists at all, and supported the abolishment of all borders. Lenin for example wanted all states of the Soviet Union, regardless of their size, to have the same say as Russia, cause borders do not matter (something that ironically was later not supported from Stalin, who unlike Lenin, wasn't even a Russian).