Serbia did start WW1, and did betray Austro-Hungary
When Serbia attacked Bulgaria and got it's ass kicked in 1885, it was AH who stood up for Serbia and demanded from Bulgaria to return occupied territories to Serbia. Almost two decades later, Serbian politics take a 180 degrees turn.
Serbia was on a conquering streak from 1912. onwards, seizing territories that didn't belong to them. During this times, they commited horrenderous atrocities against Bulgarians and Albanians. Their next aim was Bosnia and Herzegovina.
They played the next move slick like a fox. Instead of directly causing the conflict beetwen us and then, Serbian government started recruiting and training young men, instructing them on how to handle various arms, in hopes of subtly provoking AH into attacking first, which would in case of their victory, give Serbia moral upper hand that would allow then to occupy our territories, and them to be accepted by the great western forces.
One of them was Gavrilo Princip, who was convinced by Serbian officers Apis and Tankosic to assasinate Franz Ferdinand. After not accepting proposition from AH, Serbia started World War 1, causing millions to suffer just because they wanted to expand their territories. They didn't have the strength for a direct clash, so they resorted to more "subtle" methods.