Well Carly is a fictional character, but if we look at these people as though they are real my response is still the same. She is his mother...Morgan wasn't a cat or a dog, he was her child. Regardless of how much time she spent with him she clearly loved him based on her actual interactions with him and her reaction to losing him. If Carly were a real person it is likely that she would be mourning lost opportunities, you don't expect your children to die before you...the natural order of things says that your children will bury you not the other way around. The fact that Carly didn't spend as much time with her child as she should have would not change how Nelle's actions would affect her. Gaslighting Carly using her child is specifically designed to cause Carly the maximum amount of pain because Nelle knows she loves her son, misses him and will never get over his loss.