I disagree that Princip be tried for starting WWI.
I do not think you fully understand what I am saying. I am saying WWI was started as punishment for Princip's actions. The Austro-Hungarian Government decided to kill millions of people, as punishment for Princip's actions. They were not willing to sentence Princip to death for his actions.
They committed genocide against young Serbian children, but would not sentence Princip to death because he was 19 years old.
A similar thing happened to Grynszpan. Grynszpan was a Polish-Jewish 17 year old living in France who assassinated a German diplomat, sparking Kristallnacht. The German's captured him when they later invaded France. While the Germans had no problem murdering millions of Jews without any trial, they could not come up with a legal way to bring him up on trial. He lived until nearly the end of the war.