Sorry, but thats not true... why would Serbia kill a heir and cause Austrians to hit them hard and lost 50% of its population? thats just a propaganda or as I should say its a German version of WWI
Please see who was involved in that organization, there were Serbs, Muslims and Croats called "Mlada Bosna" (Jung Bosnia). And the reason for his assassination were two!
1st is that the heir was a conqueror and he openly stated for many times that Austria-Hungary should conquer Serbia! that was the reason Serbia helped them in it!
2nd is that Bosnia and Croatia wanted independence even if that means making a united country with Serbs! as they did in Corfu agreement. The reason they didn't want to live in Austria-Hungary was the reason that Germans where germanizing those lands and everything slavic was fading away, they just didn't want to loose their way of living.
The assassination was an excuse for conquering Serbia
don't forget that Austria-Hungary was an Empire, and empires always expand
like they "annexed" Bosnia from Turkey