Why is everyone killable?
This seem like odd question ...
Why should they not be killable? O_o
You are meeting mortals, and one of major feature of such creatures is being "able to die". O_o
For myself it was realy releaving that we are technicaly "able" to kill anyone and screw our story completely. :-/
I HATED it in Skyrim or new Fallout, that "cruicial" characters are just knocked down, seemed exhausted for few seconds, and them just stand up with full HP attacking once again. O_o
Its nice to see that developers are actualy creating consequences for our decisions, and they have to count with that in story creation that litteraly every character can die, not just bcs we kill it, it simply can ... its dangerous world out there. :-/
Can you imagine DM telling you
"nah, you cant attack that guy, i need him for story i prepared" or even worse
"you would kill him, but i need him for the story, so he survived three shots in the chest, decapitation, poisoning, and burning his body to dust" ... and still concidering that being a well handled plot? O_o