I don't think they'd have him kill her. Rumplestiltskin's a fan favorite, not to mention one of the few interesting characters on the show, and he'd lose his status as both if he murdered a (presumably) innocent girl like Belle in a violent rage. Not to mention the hit it could give to the ratings. OUaT is supposed to be a family friendly show, and there's nothing family-friendly about overt displays of domestic abuse.

For my own peace of mind, I'll assume that Rumple diverts his rage from Belle by destroying everything in whatever room that scene takes place in. As for what happens to her, perhaps he releases her from her contract early in the interest of the safety and sanity of both parties.