Very good film but very difficult to follow via a one-time view. You need to get to know the characters. I watched it a second time and constantly pressed the rewind button to understand what was going on. I also wrote down the names of the key characters to make things a bit easier for myself.
From what I could tell, the murder victim pricked himself on a sharp point on the ring, which contained poison. Towards the end, the murderer deliberately jabbed himself with the same ring when he realised that the game was up and would have died had the film continued a bit longer
Stamford Bridge, home of Chelsea FC, is seen at the beginning of the film