First of sorry, because I'm 100% sure this has been asked before but I just couldn't figure out how to search in a special section of the forums.

Anyway as the title says, is there any mod that makes YOU able to kill all npc's but still making them unkillable by bandits for example.

I know there is a lot of mods out there, but all they seem to do is simply removing the essential status and making it certain that half of the quest characters will die in dragon attacks.

My guess would be 'Yes' and 'No'.

If memory serves, 'Protected' status means that an NPC can only be killed by the Player, so that part is completely do-able; but as I found out myself recently, it seems most Essential NPCs are made Essential via Quest Aliases, which (so far as I can tell) cannot be over-ridden without altering the Alias itself; in theory, it should be possible, I just expect it would be a lot of work, and might introduce incompatibilities.