It's a special cleaning service I provide to the Syndi's to prevent them taking out any bounties they put on my head. 
Dey
In-House Clan Warfare
Dey did hit a good point on this issue.
(he does that now and then it seems)
Getting into a cabal requires the approval of your peers. Now you go about killing all your clanmates, great your violent, but your clanmates will hate you. All it takes is one, one clanmate getting into the cabal and your chances of approval diminish.
Now if you add to the killing, looting, and trashtalk about how buff you are...well...your not going to be very likeable. I know for a fact (as i have done it myself) that if a current cabal trusted finds your actions...distasteful...they can do many many things to keep you from getting in. Notes showing your self-serving ways, highlight that you contribute nothing to the clan and will most likely contirbute nothing to the cabal and well, the rest is history.
Now...killing all your clanmates and showing superiority might get you in sometimes, but honestly, its not the best way to earn respect.
I always found killing the members of the other clans worked better for me, lol.
There's no clear-set rule that says massacring your own clan members will get you in, there's no rule that says massacring your own clan members won't get you in. It's all in the context of the RP of your character.
Don't worry about all this "Oh man, I need to kill my clannies to get into this Cabal" or "Oh man, I better not kill this clannie or I won't get into the Cabal" crap. Just play your character out how his/her personality and RP dictates they would act, and if it involves killing a clannie, so be it.
Dey