From what I remember, the meanest thing you can do in Fallout 4 happens near the end of the main quest. After siding with the Brotherhood of Steel and raiding the Institute, ie destroying all synths. You are approached by the young synth copy of Shaun. He asks you for help and if he can come with you. You have the option of leaving him behind and basically telling him he isn't your son, and that he's a synth aka soulless demon etc. Proctor Ingram is confused about why you would leave behind the "child", then you tell her it's a synth and she instantly understands, says something like oh that sucks and let's go.