He killed one

My source is my secondary school knowledge, but I suppose these could be considered credible enough. As for the movement that actually tried uniting South Slavs, you're looking for the Illyrian Movement. The Black Hand was mostly there for Serbs outside of Serbia, and their main point was Bosnia.
encyclopedia.1914-1918-online.net/article/black_hand
britannica.com/topic/Black-Hand-secret-Serbian-society

And yet, nowhere in these sources does it say
>unite south slavs under serbia
If you meant, the creation of a south slav state under the serbian monarchy, that happened after Serbia emerged on the winner's side and was handed the territories on a plate, nothing to do with the black hand
Your secondary school knowledge is also dismissable if your government is particularly butthurt about us.

>If you meant, the creation of a south slav state under the serbian monarchy
No, I mean uniting the areas they considered to be rightful parts of Serbia, which is why their main point was Bosnia. I, admittedly, phrased it terribly because I wanted the claim to sound similar to your claim while emphasizing that the Black hand was primarily a Serbian movement. That is my fault and the source of some confusion here.
> that happened after Serbia emerged on the winner's side and was handed the territories on a plate
I know.
>Your secondary school knowledge is also dismissable if your government is particularly butthurt about us.
It really isn't, since neither of the sources I've presented is my secondary school knowledge.