I don't think Molly's "ditto" ruined it. After all, just before Sam was killed, they had been arguing over the fact that he always said,"ditto," rather than, "I love you." She said it to show him that she understood.
I think there was slightly more to Molly's "ditto" than that. Sam said it because he wasn't ready to commit. Molly said it at the end because she was ready to let him go.
I thought he had been ready to commit before Molly was. It was implied that he had asked her to marry him at some point before the movie took place but she refused (he mentioned that she had never wanted to talk about it.)
To be fair he had kept them waiting long enough. If I were the light I would have tossed his ass in purgatory for a few millennia for making a break for it the first time.
Hey! You're not old enough to drink! Now go and die for your country!!!