Basic idea that John Wilkes Booth is trying to convey through his diary is his strong belief that Lincoln is a tyrant whose assassination is the only salvation for the country.
Also, we see that he considers himself a patriot, a person willing to liberate his country at the cost of being hunted, prosecuted or killed.
So, when Booth says "Our country owed all her troubles to him..." and "And yet I, for striking down a greater tyrant..." he clearly thinks of Lincoln as of the tyrant oppressing his country.
And from " I care not what becomes of me" and " I struck for my country and that alone" we see the reason for his action and lack of concern for his life and destiny.
