Perfect plot, perfect writing, but he end of going back again is my only problem (if its not broken don't try to fix it) 

though I understand its an opening for season 2,  it would have been better if they moved to the future

While reading the comments I found one by 'luffykhi' explaining that they might have gone back to try and save the three murder victims. Knowing they'll have their son  (or at least Haejun knowing he'll have their son) to bring them back safely. After reading that, it made more sense to me. Of course they would want to save Yoon Young's aunt. 

Ofc they went back. Last time they just save the 3rd victim, soon ae and hae jun. This time they will save the other 3 victims of yeon woo.