[@Dnafein] Ok, so what happened was this:

*Sinestro is Yellow Lantern and stole 1 of each of the other Lantern Rings and took them to Earth.
*Sinestro released them on Earth with a special little modification that both masked them from other Lanterns as well as bound them to Earth, so they had to find a host on the planet.
*Martin was the first one found and he was found by the Green ring.
*Sinestro tricked Martin and the others that the rings chose into believing that they were 1 big team that weilded the 7 magic rings of power, with him as the leader. They tracked down all of the members and grew closer to each other as they did so.
*After the others were found, he tricked Mongol into attacking Earth in order to claim the Rings for himself to make him into a Lantern god of sorts. This set up a situation where the other Lanterns were forced to combine their powers and, with all ring colour weilders in agreement, they summoned the White Lantern ring and gave it to Sinestro.
*Having everything he needed, he dropped all pretenses, easily defeated Mongol, told them "By the way, i fucking hate every single one of you and thanks for the White Ring, taking War-World and going to conquer the universe now, since no-one can stop me."
*Martin attacked him, but having lost all sense of reason at this betrayal, the Green ring sensed that he was no longer fighting on will-power and abandoned him.
*Sinestro found that Martins will to fight him, even after the Green ring abandoned him, caused him for the briefest of nanoseconds the ever so slightest twinge of fear that "Good thing the Green Ring abandoned him or he might have actually killed me with that kind of willpower." It took the Yellow ring a little time, but after a few days, it picked up on that and bound itself to the one that made its former master feel fear.
*He is now looking for purpose, something to do with this power.

Sinestro has always feared what he can't control. That's why he took Arin Sur's death so badly, that's why Paralax was drawn to him and why he became the champion for yellow. He saw in Martin a great power that he couldn't control, but he let Martin live, because he was determined to prove that he had overcome that fear. Plus, they had developed a sort of "Student-teacher bond" that Sinestro didn't want to admit to, but also didn't let stand in his way. Sinestro won. That's not a great departure from his personality. This is Martin dealing with having his ass thoroughly handed to him by Sinestro.