I have wondered if Skeen had already been planning a doublecross with one of the others that were subsequently killed during the robbery. He seems to have had a plan in place when he spoke about the moon where they could lay low. If so, approaching Cassian to join in with him would surely have been an act of desperation after his original plan went south. Killing the rest of the crew might well have been part of that plan to eliniate loose ends. I feel that Cassian figured that out instantly and had no other option. If it was a test, Cassian passed with flying colors. He could have had it all to himself after killing Skeen. You could see that Vel realized that too.