Austria-Hungary had been thinking about invading Serbia for decades.
I dislike this historical view where the little guy is blamed.
OP's point is that an act can seem small on the surface ("a guy got assassinated") but triggered a larger war because there are greater tensions and forces at play ("Austria-Hungary had been thinking about invading Serbia for decades").
WWI started over an assassination that barely happened, and WWII over a city ( Danzig, although it's fairly certain Hitler wouldn't have stopped there, like WWI would have happened anyways).
WW2 so, was different. That was, from the beginning, Hitler's idea. He wanted it, he took what he could without a war, and then with a war. All with the goal of defeating Russia and exterminate the Jews (and all other undesirables). Everything else was just excuses, and thinly vailed ones at that.