The Hutt that Luke and his squadron had been sent to take out was burrowed into the ground, as they tended to do, and was directly under the support settlement it needed to let it live it's life of disgusting greed. Luke's orders had been clear- he and the other 3 TIE fighters in his squadron were to guard the 5 bombers, who were to bomb the entire settlement from above, heavily enough to take out the compound below, and some of the stormtroopers would come in after them and kill off any of the gangsters that had survived the initial bombing.
The settlement came into view as they rounded the planet, dropping lower as they approached. Luke fired on the east guardtower, ordering the other TIE pilots to take out the others, and the bombers began to move into position above the settlement. As he scanned the settlement again for further defenses, his eyes caught on the civilians fearfully running for cover- and the slaves, glancing around, uncertain. These were no enemies of the Empire, no destroyers of peace and stability. A rush of anger surged through him. Did they really expect him to order his men to bomb innocent civilians? Slaves? There had to be some way to do this with less casualties- sending stormtroopers into the base to flush the Hutt out would probably work!
He wasn't going to let this happen. Almost without thinking, he surged into action, spinning his TIE fighter to face the bombers and beginning to fire. He knew the ships his squadron flew, even if he didn't fly the bombers- he knew every weakness in the ships. Taking them down was easy, the other TIEs taking a moment to move into action against him, two of the five bombers already falling to the ground below by the time the first of the other TIE pilots had moved into action.