Is it possible to kill the members of the thieves guild and the assasins guild or are they essential?

theres a quest to kill the majority of the dark brotherhood, but you can't take down the thieves guild (but unless you join the theives guild they svck)

Well, there's some glitch about that makes you able to kill essentials. Thought it involved beating them into "submission" and then spam raise zombie spells on their face?
Could be wrong. There's a thread about it in this section, actually. Haven't read it completely.

Hmmm
Could there be a mod that allows it?

the zombie glitch is only temp(witch is a good thing) cus I rage spam then regret later but they do come back but (for me) the ash piles take ages to get cleaned