I took a look again at Velocity's story after Rafay was lynched:
Thel ‘Vadamee was furious. His initial invasion force had been mostly held off by the humans, despite his earlier assumptions. His forces were making progress, though it is far slower than anticipated. Compounding onto that, more personally, an elite team he had sent to eliminate a high value Spartan team, had perished. One of them he had known well, an honorable warrior and stoic defender of the Great Journey. That was the last straw for not just him, but also the High Prophets.
Hearing his failures, the failure to eliminate the human’s military jewel and recover the abundance of holy artifacts stored there, the Prophets themselves had him return to High Charity, for a “special ceremony.” Upon his arrival, he was put in chains, and taken to the Council hall.
Thel ‘Vadamee, a proud Fleetmaster... Reduced to a mere suicide errand boy for the Prophets.
He had become... The Arbiter.
To make matters worse for himself, the Prophet of Regret ordered him back to the human planet of Reach, to attack and eliminate the Spartan team his men had failed to do so, by himself.
Upon his arrival back to the planet, he quickly set out on his path of Vengeance against the Spartans that were the reason he was in this unfortunate situation. He has a grudge to settle, and nothing will stop him.
One could draw a conclusion (especially after reading last paragraph) that there's just one guy left. I also didn't noticed in the text any involvement of 3rd party, so I'm bit skeptic about SK just yet.
Though if he exists, Brewers is also one of the possible suspects.
For now, assuming we're talking about mafia only, I agree that Robert is a best target. But I'll let him come and talk before I vote.