You cannot complete the mission if you already talked to the person about killing all the Blueskins. This mission actually starts beforehand. After killing Rust in the single security level, there is a Blueskin in the courtyard on your way back there. Talk to him and he will congratulate you on killing Rust. Kill him when a guard is not nearby. This Blueskin cannot be found afterwards. Note: The Blueskin in the Single Max section is not on the Blueskin kill list because he dies in the cell block riot. The final Blueskin needed to complete the mission is in the upper mines. He gives you information about a keycard carrying guard you need to kill. Kill him immediately, or afterwards.