What ho, Curmudgeon? When the Austrians demanded concessions and reparations from the Serbs for shooting Ferdinand, the Serbs agreed! Yes, the Serbs were willing to meet just about every Austrian demand.
The Austrians attacked anyway, and promptly had their clocks cleaned in a series of massive defeats administered by the outgunned, outnumbered, and poverty-stricken Serbs (and Montenegrins)! Quite a campaign ... shot'em up real good.
The Czar mobilized to protect the Serbs, the Germans mobilized to bail out the Austrians, The French Mobilized to back the Czar, etc.
Now how is this the fault of the Serbs? OK, shooting Ferdinand was not a good thing, but jeez, the Serbs said they were sorry! (I can hear those Serbs now: "Relax, It was just a splinter group. Crazy Radicals. We'll pay. Wadda you Austrian guys want? Name it. It's yours") True, the Serbs (and Montenegrins) were mighty perturbed that they had just whipped the Turks and that Austria was going to grab Bosnia. But the Serbs did accept pretty darn near all of Austria's demands.
What the Serbs contributed to the origin of WWI was the agitation and trouble they stirred up among Serbs lining in the Austro/Hungarian empire on behalf of the pan-Slav movement.
The introduction of state sponsored terrorism into European balance of power, and you think this was a lark? Sad.