I'm not sure if it's a documentary. It could be a docudrama, which combines elements of a documentary with dramatic re - enactments. Or it could be a regular drama series that is inspired by a true story. Without more information, it's hard to say for certain whether it's a pure documentary or not.
One well - known case could be the Leopold and Loeb case. They were friends who committed a murder. Their case was notable because they were wealthy and intelligent young men. They thought they could get away with the crime as a sort of intellectual experiment. Their actions were highly publicized and shocked society at the time.
It could be about a true story where friends are involved in a killing. Maybe it's a story of betrayal among friends that led to a tragic end. It might explore the motives, like jealousy or greed, that could drive a friend to commit such a heinous act.
Revenge can play a role too. If one friend feels wronged by the other, maybe through an act of betrayal or a hurtful comment in the past, they could seek revenge in the most extreme way, which is murder.
Often, it can start with a shared deviant interest. For example, if two friends are both interested in violence or have a warped view of power, they might gradually escalate their actions until it leads to killing. It could also be due to one friend being highly influenced by the other, like in a cult - like situation.