I thought Marvolo was dead already. But I think you're right. Or did he get sent to Azkaban, and Merope used that time to escape from the house?
He didn't actually kill the Gaunts. Marvalo was in Azkaban and he used Morfin as a scape goat. Voldemort killed the Riddle's with Morfin's wand, planted the memory in Morfin's mind so he would think he did it, and then stole the ring. Morfin was then sent to Azkaban and died there. Marvalo died within 3 months of returning to the Gaunt house after his sentance in Azkaban as there was no one to care for him.