‘Abdel Fattah Younis, formerly Libya's Interior Minister, who defected to the rebels early in the uprising, has been assassinated in Benghazi along with two of his aides. The Transitional National Council (TNC, or sometimes NTC as the English version is sometimes given as National Transitional Council) is apparently blaming Qadhafi supporters, but it is also being reported that Younis, who has been the senior rebel military commander, had been summoned back to Benghazi to be questioned about "military matters," and there are rumors he was about to be arrested; he was killed before arriving. Some are hinting he might have still maintained ties to Qadhafi or been some kind of double agent, but so far the TNC is praising him as a hero.
The situation is far from clear, and doubtless there are many with vendettas against him from his days as Qadhafi's security man, just as the Qadhafi regime itself may have revenged itself for his defection. It's like one of those Agatha Christie tales where everybody had a motive. Twitter is rife with rumors of shots near the National Council, of this being the result of some sort of internal tribal feud, etc. Perhaps the situation will become clearer with time.
Since he was the most senior experienced military man on the rebel side, his killing will presumably be a setback for them at a time when they have been making slow (painfully slow) but steady progress.