I guess xingtu has a very deep reason why he kill her ex wife . Something like a disease he might have ? and he will not be able to accompany guan xue until they get old(which guan xue wants), that is why xingtu mastered a power to bring back the past for guan xue to meet a better man that can be with her forever. And the only way maybe to accomplish it is to kill guan xue from the present. Just a guess, since i am seeing that xingtu actually cares for her so deeply.
I guess xingtu has a very deep reason why he kill her ex wife . Something like a disease he might have ? and he will not be able to accompany guan xue until they get old(which guan xue wants), that is why xingtu mastered a power to bring back the past for guan xue to meet a better man that can be with her forever. And the only way maybe to accomplish it is to kill guan xue from the present. Just a guess, since i am seeing that xingtu actually cares for her so deeply.