The C machine is chock full of people who are useful for a while and then no more. They don’t even have to be a threat. Just the fact that they know things makes them a threat.
Basic C M.O. is use someone to perform a dirty deed that you don’t want to be tied to, then off them once the deed is done. Plenty of dead ex-aides in the wake of HRC. Also, when you get rid of them, that is one less person with whom you have to share your ill-gotten gains. One less person expecting favors from you or another job. One less person who might say something by mistake. Speaking of which, I’m surprised they allow Maxine Waters to keep breathing. She blabs constantly.
Chuck Schumer is another one who I believe has been a bit too chatty now and then for DS comfort.