I want to go on a killing rampage and i am curious. Are normally essential people still essential or is everyone now killable.

hardcoe mode does not change NPC. As far as I am aware everything except for children are killable whether hardcoe or not.

All hardcoe mode changes is that you require sleep, food and water; items like ammo and stimpacks have weight, healing occurs over time; and companions can die permanently. There might be more.