That was what Draco had bet on, Riddle’s need for self-preservation. Even he had just admitted that he wouldn’t have tried to kill Hermione anymore once they were married. Because if the rules were to hold up, his second would have executed him for violating that unspoken agreement. And apparently, that wasn’t a gamble Riddle was willing to take. So, instead, he chose this. To try and end it before the marriage could take place.