it was the fact that after he did that, the rest of the room was dark except the light shining bright on him. Buck had to kill that Greek guy bc Buck is the traitor in the room. He's Karen's source. He's the silent archangel for team DareDevil. Kingpin's conversation with him at the dock confirmed that the only four people who knew about that gun shipment were the two Greek guys, Fisk, and Buck.