"Official" Serbia (i.e. the government) has never been directly implicated in the assassination of the Archduke, but it is well known that the government was deeply infiltrated by irredentist elements which advocated the "redemption" of Serbian lands within the Austro-Hungarian Monarchy.

For example, one of the the prime leaders of the Black Hand which carried out the assassination, Dragutin Dimitrijević aka "Apis", was head of Military Intelligence in Serbia. Learning of the assassination attempt, the Prime Minister of Serbia, Nikola Pašić made a half-hearted effort to warn the Austrians, but was not only up for election but was also in sympathy with the irrendentists.

Austria's role as aggressor is a bit more muddled. Contrary to prevailing opinion, while they did ultimately endorse it, Germany didn't know the contents of Austria's ultimatum to Serbia until the day before it was issued. Contrast this with the fact that the Russians and French already knew about the ultimatum because the Austrian ambassador to Italy had briefed the Italian premier who dutifully passed its contents on to Paris and Russia, despite being a member of the Triple Alliance.

We know now that the "Balkan Inception" scenario had been decided upon by the French and Russians as the best means of destroying Germany as a military threat for the French, and as a means of getting support for seizing Constantinople for the Russians. As a result, the French President, Raymond_Poincaré who was in St. Petersburg at the time of the ultimatum, and the Russian Foreign Minister Sergei Sazonov made clear that there was no response Austria could take against Serbia in the wake of the assassination that would not lead to war. As a result the Russians advised the Serbians to reject the ultimatum, thus ensuring a war would occur and on 25 July the Russians began their secret mobilization so as to keep the Germans in the belief that the war could be localized to the Balkans. The goal of this deception was to create what the French and Russians considered a "defensive" war without looking like the aggressor so as to placate public opinion and get the British on board.

Finding themselves in a situation where none of the Triple Entente would acknowledge their right to protect their interests, Austrians had already settled on war with Serbia. For the Germans, the choice was to either support Austria and then be accused of forcing her into action, or holding back and be accused of being a poor ally.

In a rush, so have to cut it short, I can try to add more tomorrow.

*The Sleepwalkers: How Europe Went to War in 1914. By Christopher Clark

*The Russian Origins of the First World War. By Sean McMeekin